"Primarily for fic exchanges" isn't entirely accurate, though there are definitely a number of big fic exchanges such as Yuletide and Remix Redux that are hosted on the AO3. The vast majority of fic that gets posted to the AO3, however, consists of independently written works that people want to archive in a single place (as you do), including works written for fic exchanges hosted on LJ, DW, etc. I mean, there are almost 250,000 works on the AO3 right now; at most, maybe 15,000 of them were written for Yuletide (aka ~6% of the total works on the archive).

However, the AO3 does have a lot of tools to help moderators of exchanges match participants, monitor submissions, reveal works (and authors' names, in exchanges with a period of anonymity after story reveals) easily, etc., which is part of why so many exchanges are hosted there. (Personally, as a fairly frequent participant of fic exchanges, I'm happy about this trend, because I love being able to edit my stories after submission without having to bug the moderator.)