1) They make it very easy to crosspost from them to LJ. Everything I'm posting these days is being posted on DW first, which x-posts it to LJ, which is then nabbed by Facebook and published THERE as a note. So I can be read multiple places for friends who only have one thing or the other.
2) Offsite backup, baby! You can import your ENTIRE LJ, including your locked and filtered posts, AND the comments attached. The only thing that doesn't import are the icons, it posts your default one to DW. It also leaves your locks INTACT, so if it's private or filtered on LJ, the same lock or filter applies on DW. I've often worried what I would do if LJ went down for good. I'd lose a lot of good memories if LJ went down, but now I have full redundancy.