1. To have your work read, and read by as many readers as possible. Self-publishing is the best way to achieve this.
2. You can publish a book that looks professional and compares well with a traditionally published book. So do it!
3. You can update, correct and add to your books very quickly.
4. You earn more of the money. 40% -70% is a lot more than trad publishers offer.
5. You can privately, and definitely behind closed doors, cock a snook at the agents and trad publishers who 'loved' your book but couldn't sell it, especially when it is selling nicely in both e-book and print forms.
Gloat privately, but just sometimes it does feel good to see that 5 star review and know some kind reader admires your writing skills and story telling ability.