1. They know whether you’re actually getting a good deal
Since 90% of the airline tickets you buy are for a domestic flight to see your parents, your knowledge of the relative value of a flight to London is, how shall we say, limited. A travel agent, however, books, like, 74 of those a day and will know immediately if the $600 whatever website told you is a “smoldering price” is actually reasonable.
2. Travel agents know where to go in a city, or know someone who does
If you don’t happen to be headed to one of Thrillist's 30 cities, it helps to have a reputable resource available who knows the best restaurants, dive bars, or cockfighting arenas without having to sift through a thousand ridiculous reviews about how the waitress didn't bring the table enough bread. Since they book trips all the time, TA's know this stuff like the back of their hands. And if they don’t, they’re one call away from getting you an answer.