important, useful, valuable
critical, crucial, essential, key, vital
chief, main, major, primary, prime
basic
particular, specific
proper
original
dual
The committee has a dual function, both advisory and regulatory.

bodily, brain, liver, mental, etc.
Fortunately, his head injuries left his bodily functions unimpaired.

have
carry out, fulfil/fulfill, perform, serve
All members carry out their own particular functions.
The club serves a useful function as a meeting place.

charity, official, social
The couple attended a charity function in aid of cancer research.

hold
attend, go to

room
The reception will be held in the hotel's function room.