If your book sells like gangbusters (in the hundreds of thousands) Agents will contact you. As it stands now, you don't need an Agent for anything, unless as pointed out, someone wants to do a movie based on your book.
The Cicadas http://amzn.to/z9kjPG
I think agents do have a lot to offer if you need editing or are generally doubtful and need advice on your projects. However, if you have a novel you are satisfied with I recommend self-pubbing. If you have a good book, the potential is there to make a lot more money than with traditional pubbing.
I tried to do both at first but a lot of agents aren't interested after you self-pub, like the previous poster said, until your book sells. If I went with an agent at this point, it would be because they were in love with my book...not my sales figures.
You have to be very careful and selective about which agents you submit to. And even more careful if you are solicited by the agent. Reputable agents usually don't go looking for books, the books come to them.
I had two of my books agented, but they didn't sell. If I got an offer from a publisher, I would certainly solicit an agent because there's a lot to this business that we don't understand. They earn their commission
If you're talking about agents who get writers to self publish, I don't know about that. Don't trust it.
I was signed by a really good agent for my book, and although we didn't manage to sell it to the majors, the support and encouragement I received in developing my book was invaluable - the book would literally not exist without my agent encouraging me to develop it.
Even though it didn't sell, I also really benefited from getting detailed feedback from editors as opposed to form rejections. Almost all of them liked it and had very positive things to say about it, but they all felt it didn't fit neatly into one area of the market (it's kind of pitched between noir and mainstream action - both types of book I love). That was encouraging because it told me there was nothing wrong with the quality of the book, and that's why I decided it would be a good experiment for self-publishing.
I should also add that my agent was really supportive when I suggested going down this route, and is still keen to read my next book.
The only purpose of an agent is to hawk your ms to publishers. Self-publishing is an end-run around that entire process. What you DO need is an accredited editor, even if you are highly educated. Sometime in the future, you may want to go the traditional route through a publisher; then you will need an agent, who will suck 15% or more from your royalties.
The only self-publishers who use agents are the ones who also deal with publishers, and the agent has nothing to do with the self-published work.