Unless they use different emails for different websites. This sounds extreme, but you just pointed out one reason that might be a good idea.

And other reasons include:

Knowing who leaked your email or sold it to spammers

Being able to stop spam by deleting the email

You could even consider binding the email to a domain so the address would only accept emails from the domain you gave this email address.

You don't really want different mailboxes, you want one mailbox with different aliases. An alias becomes a "communication token" which can be revoked, very much like when paypal gives a payment authorisation token to an ecommerce website. If that token gets leaked, no big deal, no one else can use it. A single email address is more like a credit card number.

Try <username>+<per_site_suffix>@gmail.com and then set up a filter for every suffix.

The per_site_suffix can be anything you want.

Except that everyone does that, I am sure spammers very well know that suffix trick.

Well, the data is already on the internet...

it is more convenient, just upvote and move on