I don't see the necessity. Can you expand on that?

I think you're right not to see it. Valuing happiness is a relatively recent development in human thought. Much of ethics prior to the enlightenment dealt more with duties and following rules. In fact, seeking pleasure or happiness (particularly from food, sex, etc.) was generally looked down or actively disapproved. People may generally do what they calculate to be best, but best need not mean maximizing anything related to happiness.

Ultra-orthodox adherence to religion is probably the most obvious example of this principle, particularly Judaism, since there's no infinitely-good-heaven to obfuscate the matter. You don't follow the rules because they'll make you or others happy, you follow them because you believe it's the right thing to do.