Yes, that's exactly what I'm going to say
For IMAP, most if not all mail systems (gmail, yahoo, aol, fastmail, gmx, etc.) support server side rules.
Why do this in a mail app when you can have the server "put it on a plate", so each message is synced into the right folder the first time through?
As for POP3 -- well, some mail providers only offer POP3 and no IMAP...
Just like I suppose there are people who still listen to Billy Idol. Maybe those are same people (working at those mail providers, I mean)?