My various inboxes are constantly bombarded with emails I have zero interest in reading. Part of the problem is that spammers have gotten really good at circumventing spam filters. But another issue is that so many email clients do not enable junk filters out of the box. For anyone who suffers through the usual deluge of junk email, that's a problem.
Sure, it's not hard to just delete every irrelevant, offensive, or eye-roll-inducing email that comes along. The problem is, when such a large percentage of your email is junk, you might accidentally (and regularly) delete something that's actually important.
Also: How to use Apple Mail Redirect (and when not to)
Ergo