- it's fast (can take over a year to publish through traditional system)
- if you have the rights, you can publish your backlist
- you can get a much high percentage of royalties (eg 70% via Amazon)
- you can publish work which might sell in relatively small numbers (which wouldn't interest a mainstream publisher)
And the disadvantages:
- the work won't be in libraries and bookshops
- the work won't be reviewed in the big journals