Lots of reasons
I've used the one for over 10 years (also do my business flights through them, as did my main former client). Basically I plan nearly all my flights and hotels then just get the TA to book them (only talking about overseas here); occasionally they will come up with a better solution or deal, or at least say to me "have you thought about this ...". There are still some airline flight combinations/deals that you simply can't book on-line and when you can, TAs have the ability to request 'exceptions' from certain airfare rules direct from the airline.
Costs me no more on balance and when they can't get close to what I get on-line, they are happy for me to book those bits myself. For something like a DONE4, or some other itinerary, if you book on line, you have to pay there and then. Using a TA, I only need pay when the fare is actually pay-able (ie I think a month prior for a DONE4) and if the pay-by date is still a long time before the flight, they can check the availability, then cancel the old booking and re-book it, thus kicking the pay-date down the road (although this lurk is getting harder to do these days, as the airlines have started penalising the TAs.)
When the TA does the booking, I get a single itinerary PDF with ALL the flights, hotels etc on the one form, with all booking numbers, refs, phone #s etc. They can join different booking PNRs seamlessly; when they had booked my up-coming DONE4, and the DSC came along, they cancelled and re-booked the entire thing to get me in the DSC period. That one might have cost me a bunch of flowers (I usually end up sending at least one bunch per year
)
Lastly, if I go into pamper mode (ie luxury hotel), I ask for 'Virtuoso' benefits, as the TA is Virtuoso accredited. For a separate rate (sometimes a bit more, but once it was less!) you get a bunch of extras such as airport transfers, spa/restaurant credits, stronger- guaranteed room upgrade ("subject to availability" of course). Nearly always very much worth it (but not in Galle
).
If something goes wrong, there is a 'real' 24/7 phone number I call (or e-mail) and they then scramble to get alternate flights etc - what, me worry?
And all this is done in their time, not mine.
So, for me its pretty much a no-brainer. But I need to add that I have been a really good customer of their for ages, and I'm sure not everyone gets the service I do.