More Housecleaning

I'm definitely improving in my cleaning skills, but there's one thing I don't get. Every toilet cleaner commercial talks about cleaning under the rim, and every toilet brush has a thing specifically for cleaning under the rim. What I want to know is, why? Setting aside the question of how does anything even get under there, what's the point of cleaning there? I've been using toilets for over 50 years and I can tell you I have never once had occasion to look under the rim, and I've had plenty of experience with plumbers and even if there's a real problem with the toilet, I've never seen any of them look under the rim, or say to me, "Ya know, the problem's under the rim." Come to think of it, how would you look there even if you tried?