I used our corporate TA (CWT) in London to book a GUF2 upgrade itinerary.
I would have used our US corporate TA to book a slightly more complex itinerary where they were able to get a better price than BA (but couldn't as I had an e-voucher to redeem and could only do that via BA directly).
Apart from that I can't think of when I would use a TA for booking flights, especially personal ones.