The result is a merchant's webpage where I can't see anything for sale unless I sign up for an account. I know what that means -- they'll email bomb you with daily ads for their stuff. Even if you opt out of messages, they often still send them anyway. Worse, they'll share your email address with other merchants, some with the same corporate ownership, others not.
Soon you're getting a dozen emails a day from companies you've never heard of nor visited their webpage. Their ads also now appear in many of the other websites you visit, because they put tracking cookies on your browser. Finding these cookies to remove is often a bitch because they use a different domain name.
I circumvent this by using a throwaway email account only for this purpose. Occasionally I have to check it for a verification link. I open the box and see it's now receiving dozens of emails a day. I've provided that email address only four or five times, not forty-five times.
Apparently some retail web marketing gurus don't realize the different between selling to people and harassing them. Who wants to buy your shit when you vomit an endless stream of spam on them?