AOSP is not "useless own its own". I don't know how you got that idea. Custom ROMs are every bit as capable as Google-flavored Android.
I believe Lineage and co have to supply their own replacements for some important features, and/or use the proprietary Google Play Services.
AOSP isn't a usable base product, like Chromium for example.
(That's my understanding anyway. It's quite possible I'm wrong!)