Technically the "Learn from this App" feature isn't an Apple Intelligence feature. It's been there since at least iOS 15 in 2021. I haven't heard people really discuss it at all until recently.
My understanding is that it basically just enables suggestions. So, ordering things on your share sheet according to usage, search suggestions, the little button in Maps that navigates you to the location of your next calendar event, etc.
It could still really benefit from a "turn off all" button, though.