me+folder@example.com
Maps to the account me and will (if configured correctly) put the mail in a folder called folder if such exists.
The reason you might want many accounts with the same email seem many to me if you don't realise that you can create arbitrary distinct emails this easily.
It seems to me like all benefits of the "exact same email, multiple accounts" feature are vastly outweighed by the inconvenience for users simply forgetting that they already have an account, and creating a second one by accident that way.
I mean, even I end up almost creating an account by accident every now and then (mostly on sites using the horrible "signup is the default, login needs one additional click" pattern), and I do so using autofill from a password manager!
Sometimes you can't even contact Customer Services because "your account doesn't exist" (because you cannot feed the correct email address to their customer service site).
Thankfully it's rare, but when it happens it's extremely infuriating.
Gmail supports it. Microsoft does not. Neither does Yahoo/AOL. It likely was not widely supported in the 90s either. It’s a nice hack but it doesn’t solve every problem.