A lot of users have tons of aliases and want to categorize them into folders.

I have a lot of aliases and the main reason I want folders is to put in inactive aliases. I like disabling the aliases from accounts I delete instead of completely deleting them and it's a mess.

You don't have folders, can't set the 'Active' tab to be the default one so at least you don't see the inactive ones in the same list, and the inactive aliases stay in the same place instead of dropping to the bottom of the list.

I believe folders are the perfect feature to solve this issue and I've seen a lot of comments in Reddit threads discussing the need of folders for their own reasons.

Folders in the left panel under 'Mailboxes' would be perfect.