Why does Mac OS include both ABC and US keyboard layouts when the two perform the same functions?
As per this Apple Discussion:

You should use US or ABC.  ABC is just the same as US but you don't have an American flag.  Some users outside the US may find that preferable.
ABC-Extended (formerly called US Extended) is much like US or ABC, but it lets you type a whole lot more accented characters via option key shortcuts.  If you have a need to type Chinese pinyin with tone marks, then ABC-Extended is the one to pick.

Apple also provides ABC QWERTZ and ABC AZERTY keyboards, which are the same as the ones called German and French, but without the German and French flags, for users who prefer that.