What does the 'email' cookie do?

The first cookie in the list: ‘email’ - what does it precisely do?

According to Sam, it is “Used during account creation”

Name Essential Expires Description
email Y Session Used during account creation

I’m writing a cookie policy and would love a little more clarification. Is it used for availability checks (username/email already in use)? Does it spare the user of hassle of having to re-enter the e-mail a second time if a desired username is already taken? Thanks for your clarification!