Apple probably tested out a bunch of similar tech and liked this one the most. Not obvious how it's any better/nicer than PostgreSQL (it can do more than you'd think; Apple already uses it everywhere).
My initial guess is it's for their LLM AI; to remember (in an organized, searchable form) all the "personal context" information it absorbs. IIRC, LLM tech is still struggling to add long-term memory store, so it's probably being delegated to a tech with strong synergy.
There may be some VR applications as well, for managing virtual objects.
Not too familiar with Swift, but maybe they're adding a(nother) native db class, kinda like how Javascript has IndexedDB. Would be great for developers across the board.
Apple definitely acquired them to avoid future drama; not wise to have a critical part of your business be external (*cough* manufacturing *cough*)