I travel often, here's my take. I usually do all the work myself, I don't use travel agent much. However, a travel agent is good for a few reasons:
1, for booking tours i.e. Cosmos, or a cruise. It's better to use them. They make all the arrangements.
2, when you find a flight that you like, but don't want to book right away, a travel agent can hold the spot for you for 24 hour. Last year, I found the flight to Morocco that, believe it or not, fluctuated like stock market. The price changed a few time in a day. My travel agent told me that he could hold the rate for me for 24hr, and that night, if it went down, I could go ahead and book it on my own. If it went up, then he could book for me using his rate the next day. The rate went up overnight, but I was fine cause he reserved the rate for me.
3, discount! My travel agent usually gives me a discount. Not much, but it's better than nothing.