Because to some people, correct proportions are more important than accuracy to the original display. For instance, if you play Kung Fu Master and many other games in full 4x3 you'll notice things looking very squashed, circles become ovals etc. Whereas if you play in the correct ratio, everything looks proportionally correct. Personally I prefer the latter.
I agree that this setup doesn't work for CPS games but that's why it's great that HK$ added different options for this setting.
I see.
I guess I hadn't played any games where I noticed issues, but yeah...
It's definitely nice to have the options. Then everyone can be happy.