About Plack::Middleware::Session, Do we really need to store the sessions?

About Plack::Middleware::Session, Do we really need to store the sessions?

In the builder part of a psgi app, I use the following code which I
remember seeing somewhere.
enable 'Plack::Middleware::Session',
store => Plack::Session::Store::DBI->new(
get_dbh => #....
),
state => Plack::Session::State::Cookie->new(
session_key => 'id_session',
httponly => 1,
);
1) Why do people store the sessions? And store the sessions in the server
side? 2) What happens if no sessions are stored?
I have a psgi running, where I used 'Plack::Middleware::Session' without
knowing its real purpose and now my sessions table is approaching 500
megas, so I really want to erase this if there is no need.