[scalem X]

I'm sure you'll get quite some replies to this thread from those who have done self publishing, but while we're waiting I'll sum up the reasons I've heard of:

1 -They just want their novel out there (for themselves, family and friends).
2 -They have a channel through which they can reach a decent public and therefore don't really have to bother with publishers (like if you attend fairs every weekend and can sell there about three books every week or so)
3 -Authors who self publish out of print books to supply to people who ask them.
4 -Charlatans who want to sell books with rubbish, with or without a free lucky charm.
5 -Specialized books, which no Editor will even consider because they are so specialized, but you know the circuit. (alse see number 2)

and many others.