Travel agents handle problems. If the airlines mess up the frequent flyer miles, they get it fixed and save me the headaches. If a hotel loses a reservation or there is any problem, you call them and they handle it quickly and call you back to let you know it is fixed. My agent even gives me her home and cell numbers.
They work fast. They appreciate it when people call and say, "I want blah, blah, checking in this date, checking out this date. Call me, thanks." With Disney you have to be on the phone forever.
They get you cheap prices (if they're good), and are super-nice.
If you kick them back a little extra, they can usually get you a free upgrade.
They deliver. I love that. Once I called and said I had to leave in three hours, and they had all my documents delivered in 45 minutes.
Again, they are super-nice.