What's the difference between unarmored and adventurer's clothing?

Question

From the armor table, the only difference I can see between being unarmored and wearing adventurer's clothing is that the latter imposes a dex cap (+5) while the former does not. Yes, the latter also has the Comfort armor trait, but given that describes armor being sufficiently comfortable that you can rest normally while wearing it, I'd imagine "nothing" is also fairly comfortable.

So what's the purpose of adventurer's clothing? Does it have any impact at all on environmental effects? If a monk has a +6 dex modifier, should they just run around in their birthday suits?

Is it only that adventurer's clothing can have potency runes, while "unarmored" cannot?

Answer 1

Clothing can have runes, but unarmored cannot.

As you pointed out, Unarmored and Explorer's Clothing provide the same AC bonus (+0). The clothing costs 1 sp, weighs light bulk, and has the Comfort trait.

The difference is that in order to attach defensive runes (such as potency or resilient), the rune needs to be attached to an item. That's possible with Explorer's Clothing, but not with Unarmored.

From the section on runes:

Each rune can be etched into a specific type of armor or weapon, as indicated in the Usage entry of the rune’s stat block. Explorer’s clothing can have armor runes etched on it even though it’s not armor, but because it’s not in the light, medium, or heavy armor category, it can’t have runes requiring any of those categories.

Note that Explorer's Clothing doesn't count as light armor, so it may not meet the requirements for certain armor runes.

Answer 2

While MikeQ's answer is excellent and addresses the question, you also ask

If a monk has a +6 dex modifier, should they just run around in their birthday suits?

The (current) maximum attainable dexterity (from a standard start) is 24, for a modifier of +7 (apex item, start at 18 and always boost dexterity). For 21 or less dex, when your modifier is +5 or less, it's trivial to see that wearing explorer's clothing is better (or at least not worse). At +6 dex, you break even with wearing +1 explorer's clothing (but you'll likely have +2 or +3 explorer's clothing at this point). At +7 dexterity, it will break even with +2 explorer's clothing, or be exceeded by +3 explorer's clothing, which you will likely have.

But what about other ways to get the item bonus to AC?

As far as I am aware (and at the time of writing), there are three other ways to get the item bonus to AC - Bracers of Armor, Mage Armor, and Drakeheart Mutagens, all of which come with a Dex cap as well (+5, +5, and +2, respectively). So, unless you are a really high level and only have access to +1 or worse armor, it is better to wear your explorer's clothing.