Confusion about `"use server"`

I have a few questions related to

"use server"

"use server"

:

1. Should all server-only files be

"use server"

"use server"

? If not, why not?
2. I see that you can export anything other than a function from a module that is tagged with

"use server"

"use server"

, what's the reason for it?
3. Does

"use server"

"use server"

on a module expose all functions from that module to the public (e.g. they can be called from the outside in a RPC fashion)?
4. Should I only mark

"use server"

"use server"

functions that I intend to call on the client?