The correct answer to this question is To confirm that the email you entered is yours. Without this confirmation step, anyone could create an account with someone else's email and pass it on as their own. This could lead to scammers and theft. For example, if there was no confirmation step, someone could create an account with a stranger's email.
Once the account is created on a site like FaceBook, the person could use their name and post pictures posing as them. They could even go as far as adding their real-life friends and creating a different persona from who they really are.