T-tails are commonly used on military transports that may need to do extremely short takeoffs/landings. Being so high above centerline they give more leverage than a lower mounted tail.

T-tails were seen on early jet fighter designs to, for the same effect plus it gets the elevator out of any turbulence caused by the rest of the plane. Hans Multhopp (from Focke Wulf) in Germany in WWII helped pioneer them (designed the TA-183).