Say I want to comment on a news article and need to register. I don't want random-newspaper.com to have anything directly related to my person, including anything @mydomain.com. So I quickly punch in random-email@mailinator.com to register and once I'm done I can either forget the site and email ever existed or keep using it since it's non critical and losing access to it doesn't matter.

Ideally I would have different email addresses for every site so I can keep those identities separated and free of any personal information. Last time I used it like this was probably a decade ago because since then more and more sites starting rejecting @mailinator.com addresses. I found another such solution that I have been using for the past years but this is also going the same way (not a big issue yet).