When I install a third-party app and use it, I’m making an active choice to be their user too.
When that third-party app embeds the Facebook SDK which tracks me, I don’t know about it and do not have the ability to consent to Facebook tracking me.
ATT brings Facebook to the same level as Apple and the third-party app developers by giving me the visibility and choice I would otherwise be deprived of. It should be possible to opt out of being a Facebook user. Being silently opted-in without consent is what ATT fixes.