Do you want to go out for dinner?
Shall we meet for lunch?
The whole team is meeting on Saturday for brunch.
I'm going back to the hotel for dinner.
If it helps, you can imagine that "for" reads "to have".
Note that there is an error in your thread title. You should either say "Why do we use "for" in this sentence?" or "Why is "for" used in this sentence?" You cannot say "Why we used "for" in this sentence?"