SkunkyArt | MarcinG1 — Model 3d : Concept glider MGA1T 1 of 3

The placement on top of the vertical gives it more leverage, especially with a swept tail.

Depending on wing location, it stays in undisturbed flow in a stall. Note: This is really depending on the details, the HFB-320 had a forward swept wing and a T-tail, which made a deep stall possible (and in one case fatal).

By designing the junction with the vertical well, the T-tail has less interference drag. It also helps to reduce wave drag, especially when using a well designed Küchemann body (the round, long, spiky thing on the tail junction of a Tu-154) by stretching the structure lengthwise.

It can help to increase the effectiveness of the vertical tail by keeping the air on both sides of it separated. At the other end, the fuselage does this already, so moving the horizontal tail up does not hurt so much there. As a consequence, the tail can be built lower.