System Reference Documents are for what they say on the tin: they're built as a general reference document for the game system. What goes into them exactly depends on what the authors decided to put in there to build that reference — there's no set standard. Some SRDs are enormously comprehensive libraries of just about everything or even the full text of the game, some are far more limited in coverage.
Primarily, SRDs get targeted at two audiences:
- The players themselves, to increase accessibility of the game rules. (Why they'd do that is covered further down.)
- Authors who will develop additional works for the game. These authors will use SRDs to look things up, and copy and paste material where necessary. Having an SRD available makes their work much easier.
The D&D / d20 SRD is not the only one, so Wikipedia's out of date in talking like it is, though it's certainly the most well known. It existed so third party publishers could legally release a ton of d20 system material, and know what they could legally use. Wizards created the Open Gaming License specifically for releasing the d20 SRD under it.
However, you've come across other SRDs and you're curious about them.
Why do they exist if you could just play the whole game with them?
Even if that were not the case, from the ones I've read, you could largely figure out how to play from SRDs alone, so why do they exist?
In the d20 SRD's case, they left out the character creation rules and level progression table so as to hopefully force you to buy a book.
However, for other games, being able to figure out everything is sometimes the point! For these though, it's more a matter of: why would you release a whole game for free!? (I'll talk more about that later.)
The Fate SRD publishes the entirety of Fate Core & Accelerated (plus the System Toolkit, an accessory to them). Those games are both completely free, released under Creative Commons and the OGL and also available in full as PDFs. The SRD site itself was fan-made as a channel for accessing the game in full. If you want to play this game, get yourself the PDF or open the SRD, and go play it and have fun.
Pelgrane Press offers a Gumshoe SRD. Just like Fate Core, it's the full game, free under CC and OGL.
Other times, like in the case of Gregory Woolfe's Traveller SRD, the SRD is just an unofficial fan creation to help wrangle with the system. Probably these shouldn't render buying the book unnecessary when the game's not free — sites that do this tend to receive cease & desist orders if the publisher takes umbrage.
What are the SRD creators getting out of them?
(Some people are just happy releasing things for free and that's an end in itself. However, for this I'm going to talk about the cases assuming someone gets something out of it, and that monetary gain is a longterm goal.)
When made by the owners: the right free stuff means marketing.
So as I mentioned, often this is more about why you'd release a game for free at all. Fred Hicks, cofounder of Evil Hat and creator of Fate, talked about this in his blog. I'm trimming it heavily to the key points, but the whole thing's worth reading if you're interested in this:
As a marketing strategy, audience is the goal of Pay What You Want, not sales dollars. You do this because you want more folks. (...) Engagement drives to the heart of the marketing strategy, of course, so that’s a positive feedback loop in the mix.
To pursue the sales motive in a Pay What You Want context, the audience you build should get sold follow-on materials which are not PWYW, eventually, if not immediately.
If you’re not planning on creating and selling additional things based on or tying into your PWYW product, it’s not so hot as a strategy for sales. (...) For Fate Core, it’s a bit of a no-brainer for us: we’ve got the Fate System Toolkit, Fate Worlds, the Atomic Robo RPG and a bunch of other things in the pipeline for the next two years at least. Most (perhaps all) of those aren’t gonna go out there as PWYW products in digital. On top of that, there’s the physical products as well. PWYW — especially the free aspect of it — provides a low cost try before you buy option for the purchaser who eventually intends to buy the paper version of the game.
Evil Hat gets a large portion of income from Fate being implemented in other games. In a public Evil Hat quarterly sales report, Fred Hicks was surprised Fate Core actually made them money at all:
Probably the biggest stand-out stat we’ve got right now is the performance of our Pay What You Want trio: Fate Core, Fate Accelerated, and Fate System Toolkit (Ed: Since the blog post quoted above, the Toolkit turned from a sales item into pay-what-you-want including free). Since the Pay What You Want model debuted on DriveThruRPG as part of the launch of the Fate Core PDF there this summer, I’ve been opting to track straight up the amount of money contributed rather than the number of downloads paid or otherwise: it seems to be a solid measure of success where PWYW is concerned, since we’re essentially answering the question “How much was the Fate audience willing to pay for something they could get for free?” The answer’s pretty surprising: in just a few short months, across those three products, we’ve seen people pay over $9,000 for PDF content they could otherwise get free.
They released it to get people using the game and get it attention. Making any money directly from providing this material was a bonus — the point is the money that arrives later as a side effect of the free material.
Pelgrane Press is in a similar situation with Gumshoe, which has been extended into Trail of Cthulhu and other investigative games. They don't need to sell Gumshoe itself — if they can get people interested in Gumshoe (because they can actually try it out for free), those people may get interested in other Gumshoe products and buy those, and spread excited word of mouth. This is good!
Worth noting that in Gumshoe's case, the SRD was released as a result of the Hillfolk kickstarter in 2013, when Gumshoe had already been out for several years and sold plenty enough.
In the case of D&D, it was released because Wizard was happy to see other people building their brand and solidifying their status as owning the go-to RPG system (d20 or D&D). This surge in reputation handily translates to money if you can take advantage of it, and Wizards could.
When made by fans: better organisation, and helping other players.
We're social animals. Generally, people like helping people. And sometimes the books we're dealing with are a pain in the arse and we really wish they had a search function or we could link to their pages, and sometimes the people thinking this have the will and means of making it so.
That gives us sites like the Fate and Traveller SRDs, to help us navigate the games we love. (Heck, the Fate SRD is transforming into a fantastic portal for Fate Core, and the author even won a silver ENnie award for their efforts on the site.)
The d20 Pathfinder SRD fan site was also originally just a far more comprehensive version of Paizo's official Pathfinder Reference Document that included all the third party stuff. Nowadays to help pay for hosting, it also operates as a store and advertises that fact all over its pages, so you could say that offering that SRD is now marketing for the store as well.
So, in summary: they contain whatever the author decides you need to be able to reference for a system. They're created and released because someone wanted their brand built or a diehard fan decided their favourite game needed one.