Some people (incorrectly) conflate Jeremy Crawford's rulings with the rules because he is empowered to make official rulings
Who is Jeremy Crawford?
Jeremy Crawford is, according to his Twitter bio:
Lead rules designer of Dungeons & Dragons, lead designer of the Player's Handbook, and the game's managing editor
Crawford's tweets are no longer official as of 1/30/19
Previously, the Sage Advice Compendium has said this concerning the sources of official1 rulings:
Official rulings on how to interpret unclear rules are made in Sage Advice. The public statements of the D&D team, or anyone else at Wizards of the Coast, are not official rulings; they are advice. One exception: the game’s rules manager, Jeremy Crawford (@JeremyECrawford), can make official rulings and usually does so in Sage Advice and on Twitter.
However, the most recent version has changed this subtly but significantly to say:
Official rulings on how to interpret rules are made here in the Sage
Advice Compendium by the game’s lead rules designer, Jeremy Crawford
(@JeremyECrawford on Twitter). The public statements of the D&D team,
or anyone else at Wizards of the Coast, are not official rulings;
they are advice. Jeremy Crawford’s tweets are often a preview of rulings that will appear here.
This is saying that no longer are the tweets of any of the WotC staff considered official, including Crawford. Tweets are now "advice" and/or "preview [s] of [official] rulings". Instead, official rulings are now only found in the published Sage Advice Compendium.
JC has confirmed this change, ironically, on Twitter:
Q: It looks like the Invoke Tweet of Jeremy Crawford feature has been
nerfed. Official rulings are now found in the Compendium only, with
this account merely providing a "preview" of possible future ones.
JC: Yes, I decided I don't want people feeling they need to dig
through my tweets for official answers.
What about old Tweets?
This of course implies that even past tweets are no longer considered official. Presumably, Crawford has moved all the old tweets he wants to be considered official to the document (many new rulings from old tweets were added in the update). That is, after all, the entire reason he downgraded them: he wanted people to have only one easily accessible source to go to to find official rulings. Given his ability to add rulings to it at any time and from however long ago, this means that old tweets are simply no longer considered official.
Crawford's Tweets/rulings have never been Rules as Written, even when official
What are the Rules as Written (RAW)?
Rules as Written (RAW) are, very simply, the text of the rules as they are printed on the pages of official books and their errata. Nothing else. It is the words and text of the rules devoid of any outside influence.2
Here is what Crawford himself has to say about this in the Sage Advice Compendium:
When I answer rules questions, I often come at them from one to three different perspectives.
RAW. “Rules as written”—that’s what RAW stands for. When I dwell on
the RAW interpretation of a rule, I’m studying what the text says in
context, without regard to the designers’ intent. The text is forced
to stand on its own. Whenever I consider a rule, I start with this
perspective; it’s important for me to see what you see, not what I
wished we’d published or thought we’d published.
Crawford's rulings are interpretations but not RAW
First let's start with Crawford's own view on the matter as expressed in this Tweet:
Q: Does something become RAW simply because you say it on Twitter, or is RAW only what's actually in the books?
JC: Official rules are in rulebooks. On Twitter and in Sage Advice, I give rulings and clarifications. The DM decides what to do with them.
In other words, he is saying that, no, his Tweets and other rulings are not Rules as Written-- they are rulings. The rules you find in the book and the ruling he is making is interpreting those rules.
The distinction is even made in the Sage Advice Compendium:
Official rulings on how to interpret unclear rules are made in Sage Advice.
But if Crawford's ruling being "official" don't mean that they are RAW what does it mean?
So, how are Crawford's rulings intended to be used?
All Crawford rulings have the same purposes whether they are official or not.
To figure out Rules as Intended (RAI)
Sometimes groups or players prefer to play according to the way the designers intended the rules to play. And so, all of these Crawford rulings are essentially revealing that. Even the RAW interpretations that he gives are, in a way, a view into what was intended since they are revealing their intended interpretation.
Crawford talks about this in this Tweet:
That's the purpose of my official rulings: a window into what we intended. The DM always has the final say.
To provide optional guidance and tools for DMs
Sometimes a DM just doesn't have a good read on how to interpret a rule at all or can't make a decision as to which way to rule. This is the main thing that Crawford is trying to provide in giving official rulings: help for DMs who want advice or clarification with rulings in their games. Crawford says as much in this Tweet:
Official rulings are about helping everyone use the rules in as straightforward a way as possible. We don't balance the game through rulings. We clarify things in an effort to make it easier for DMs to adjudicate the game.
Because, in the end, according to RAW and Crawford, the DM is the one who has complete power over the rules at their table
Rules and official rulings are tools for DMs. The DM decides how to use them in service to a particular group's tastes. link
1 - "Official" doesn't really even mean that much. What "official" means, as best as anybody can tell, is simply that when Crawford makes an interpretation of the RAW or clarifies RAI, he is speaking for all of WotC. However, that simply means that there is going to be no contradicting voices from WotC's side. This has been apparent several times when Crawford has contradicted fellow WotC employee Mike Mearls' interpretations of the rules and saying that they are not official. What it does not mean is that Crawford's interpretations are superior to anyone else's ruling by nature of being made by him.
In fact, Crawford openly agrees that his rulings are not the only or best ones to use for every person in this Tweet:
House rules—the search for what's fun for a particular group—do, indeed, overrule official rulings.
A group is welcome to use whatever ruling it likes, whether or not the ruling is official.
Even if my ruling was different from yours, you would still be right in the game you're DMing!
2 - Most of the time when you see discussions about RAW it is when someone is interpreting the RAW. In other words, they are reading the RAW and interpreting it to apply it to a whatever specific problem or issue they are trying to resolve. It is in fact, very difficult to talk about or discuss RAW without putting it through some lense of interpretation. Thus, most of the time you see RAW around (unless they are simply quoting the book) is a RAW interpretation. Sometimes we call these interpretations "rulings". Rulings are interpretations of rules.
This is an important distinction because the rules are composed of words and words and sentences often have multiple ways to be read (interpreted). Thus, it is entirely possible to have multiple valid readings or interpretations of RAW. You and I can look at the same passage and read it different ways completely, but have neither be wrong (there might be a better or worse interpretation however). It happens basically every day on this site.