DoSing the users they are signing up would be my guess. Many sites do not even do a validation step and will immediately add any submitted email to the mailing list. Your email box is then filled with tons of emails and manually unsubscribing takes a lot of effort and still only reduces the volume of incoming emails

This could just be used on its own as a way to annoy someone. Or it could be used to try to hide some legitimate email that you will receive, like if they are compromising your account somewhere that will notify you of the new login and hope you don't notice in the sea of spam.