[erlang-questions] When to use gen_server?

[eigenfunction]

> If you don't need to serialize your data
> processing requests, don't bother with all that.

Could you pls elaborate more on that? I, Like the OP , sometimes
have a hard time making a difference between a gen_server and a normal
process,
and i end up using normal processes and think that i am doing
something wrong.
Can anyone give an example of a case where a gen_server is more useful
than a normal
process?