Reasons can include:
1. getting the tail into the 'right' area of the downwash field behind the wing while respecting structural constraints over the attachment at the root (e.g. Hawk/harrier)
2. reducing loads under sideslip conditions (applies rather more to T tails)
3. local effects similar to wing dihedral concerns i.e. adjusting the flow in sideslip to perhaps 'protect' fin effectiveness