There are a lot of you out there archiving their e-mails in different folders for each customer/project or even for each person you’re communicating with. That’s a lot smarter than just leaving them in inbox. But why folder them?
I assume it’s an old habit since e-mail were new and didn’t have (good) search functions. If you’ve got an email from Paul Random, you’ll find it easier if you search for his name in the search box than open up his folder to search.
But even more important. To just hit “archive” button instead of searching for his folder when you’re archiving his email will save you a lot of seconds each time. 20 seconds 50 times a day, it’s more than two hours a week or 100 hours a year. Plus the time you save while you’re searching.