Those buttons are the menu. The idea of menus on watches is a modern ux extravagance.

I don’t have the satisfying answer as to why they dedicated a button to 12/24 hour time but my guess is that it’s simply a cool digital feature and stands out against the limitations of analog watches.

They had modal menus back then, they just didn't display them. You had to memorize which obscure symbols to press, to change various settings. For example, in the 1970's, HP's calculator watch required pressing ⯅• to switch between 12 hr and 24 hr modes, or A⯅A to clear an alarm.

In the 80's and 90's, when sixteen-segment and dot-matrix displays made displaying text easier, calculators started displaying menu items, so you didn't have to memorize the sequence.

Since then, we've moved on to full-color high-resolution displays that can handle any text of any language, and the menus have more-or-less switched back to memorizing a sequence of which obscure symbols to press, I guess because some people think they look cool? Sometimes there's no menu at all, and you just have to memorize finger movements.

To add to it, I always kept the manuals around, because remembering the key press sequences was akin to doing combos in arcade fighting games.

A few months without doing them, and it was hopeless trying to remember how to do them again.