A lot of negative things have been said about indie publishing, in particular that it has flooded the market with poorly written, poorly edited, substandard books that make it nearly impossible for the truly talented to stand out, and that serious writers should stick to traditional publishing.
It's true that back when traditional publishing dominated 100% of the book market, it ensured certain quality control - a book that had to pass through agents, editors and publishers to get out there had to adhere to certain standards. But on the other hand, many great books that were like a square block trying to fit in a round hole didn't get a chance at all and had to remain in the drawer (sometimes being discovered as masterpieces long years later, even after the author's death). No one is infallible or a perfect judge of what makes a good/successful book (as the publishers who had rejected Harry Potter can testify).
Personally, I see at least three main points that, in my opinion, justify the existence of indie publishing:
1. Higher profit - in some cases, writers/authors specializing in a niche market have their own following and their own access to a readers' platform they can connect with, for example blogging for a major website. Or maybe they have a real knack for marketing and promotion and want to give it a go doing things by themselves. If so, they might have no need looking for a go-between that would get most of the profit - they can do publishing on demand through CreateSpace and Kindle and reap much higher royalty (even if, in absolute numbers, authors published by major houses still earn a lot more).
2. Speed - a writer bypassing the usually long and tedious querying process, which is then followed by the long and tedious process of the agent submitting the book to various publishers, can get their work out to the market a lot more quickly. Again, this is especially great for writers of a niche market a big publisher wouldn't have bothered with.
3. Everyone gets a chance - For me, that's the bottom line. With indie publishing, everyone gets a chance - maybe not as good as they would have with traditional publishing, but at least you don't have to spend your whole life beating against the glass gates guarded by agents and publishers. If you genuinely feel you have a great, overlooked book, you can still get it out there.