Um. Well, I can give you *my* reasons for liking it: a) I suck at keeping my own fandom site up-to-date (not that I'm much better with Skyehawke, but I'm *slightly* better); b) it has review boards and hit counters and a rating system, etc, for ease of feedback whoring; c) I like having it all hosted somewhere externally, in case my site should go ker-flooey, or I should drop out of fandom, the links wouldn't rot as fast; and if the *archive* goes ker-flooey, I can blame somebody else; d) archiving a fic at an external site just feels more like "publishing" it than putting it on my own webspace-- it feels more finished, and I feel more legit about linking people there.

There's also a slightly greater chance that people will stumble across your stuff, although admittedly on Skyehawke for non-hp stuff it doesn't happen too much, it still happens more than on my own site.