I remember I went to a student-travel expert when I was planning for my first trip overseas by myself and she was awesome. She helped me to find all kinds of good deals and put together my trip, then basically took me aside and advised that I find the fares for travel inside the States on my own online.
She explained that there were a couple of really cheap airlines that she couldn't book because they didn't give a percentage to travel agents, but that they would be much cheaper than if I tried to book other flights through her.
And she was absolutely right, I saved a few hundred dollars with that tip which she didn't have to give me. I'll always think fondly of her for that, and that's the reason I always try to use a travel agent, even if I'm fairly sure where I want to go and how much it's going to cost.