We didn't want too many fake accounts. We didn't ask for phone numbers. It's very easy to get a burner email, in case someone wanted to avoid giving the main email. Burner phone numbers are harder.

Also, an important use is password and username recovery. We even got password or username request 30 minutes after signup! They had quiz to solve if they want to help during studding and it's good to track them.

We had a lot of wrong emails, in particular it was common someone@yahoo.com instead of someone@yahoo.com.ar because Yahoo! offer both options. Also someone@gmail.com.ar that does not exist, but that never stop users.

(If it help, we never asked to confirm the email again after the registration.)