1. What is it for?
(What is the meaning of this one? Some other similar expressions, please? What is the suitable answer to the question?)
2. Is this seat taken?
3. Has anybody taken this seat?
(Is #2 the active voice form of #3?)
1. What is the purpose of it.
For example: "It is used to make holes in the earth."
2. "Is this seat taken?" is the PASSIVE form. The verb "taken" is a state, not an action.
3. This is the ACTIVE form. Anybody "has taken".