1. Used to indicate the use of something.
Example:
a.This place is for exhibitions and shows.
b.I baked a cake for your birthday.

2. Used to mean because of.
Example:
a.I am so happy for you.
b.We feel deeply sorry for your loss.

3. Used to indicate time or duration.
Example:
a.He’s been famous for many decades.
b.This is all I have for today.