Well for one thing you are providing your email address to a website, which might later sell it as a "high quality" i.e. personal email to receive SPAM.

By contrast, when I sign up to a service, I can start out using some throwaway email or a even mailinator.com email. Here, I have to put the ones I care to protect.

This is a bit like those services that say "enter your domain name and we'll check if it is registered" and then front-run you in registering it!