The easiest example is for tanks. Most of the time, in 8-man content, you have a Main Tank and an Off Tank. And it's common sense that those two don't need the same thing. Thus, wether you prefer playing as a MT or an OT, maybe you'd like to build your character towards it, and you could have a skill tree where one branch gives more edge to MT, and one, to OT.
This is obviated with Tank Stance and DPS stance, which are toggled depending on if the person is main or off tank.