Set up the Environment

why does we need to alter the existing /auth/github/callback route to set the req.session.user_id = req.user.id?please i need a little explanation.
i am not having trouble passing any challenge,but advanced node and express seems a little complicated and need to practice repeatedly,any advice would be appreciated,thanks in advance.
here is the link to challenge:set-up-the-environment

I believe the point is to store the user id before the redirect. By storing it to the session, it can be referenced later.