Usually they go from 1/2 to 1/2, or 3/8 to 3/8, etc. They are in effect a short, thick extension that ratchets. The T-handle I can see. And maybe before the first flex head ratchet I could see the use with a breaker bar. Possibly a breaker and a ratcheting adapter could get into different places than a flex head ratchet could, much like a roto-head (?) ratchet might, as the flex is inline with the socket where as on a flex ratchet the flex is a short distance offset. I think that is a strech for a reason to buy one though.
I will say as someone that buys and resells, I get quite a few of them and they are fairly easy to sell. I don't keep one in my box though.