When I started thinking about submissions, I also asked myself why I needed an agent.
Then I thought about it terms of every other business on the planet. If I need to put an addition onto my house, I'd hire a contractor, not do it myself--I just don't have the expertise to do it. I could learn, just as I could learn all of the ins and outs of publishing. But that takes time that I could otherwise devote to writing.
If you don't get an agent, then you have to negotiate your contract on your own. If you are a lawyer who specializes in publishing contracts, then this should be no problem. Otherwise, you should probably find a lawyer with experience in that field. The last thing you want to do is sell your book and then find out you've given away some other rights because you didn't properly interpret the fine print....