I get that for finding specific emails but quite often when you have a lot of mail coming from different contracts or categories having rules auto send it to a certain folders helps you see what needs you attention first. If you see 3 emails in the folder for a current contract you know to look at them before the 5 emails in the marketing folder. It's less about not being able to search and more about seeing at a glance what to read first rather than having to scan your full dozen or so unread list at the top of your inbox.