Earlier quoted context omitted.
If you want the Ad optimisation from FB/Google/whoever to work, it's important. More generally, if you want to be able to track your own users and where they come from, it matters. If all of your traffic is organic, then it's irrelevant.
So if there's SKAdNetwork attribution, why do I need to give the unique device ID to FB or Google in addition to that? They will know which ad led to the install via SKAdNetwork calls anyway, right? I know IDFA benefits them because they can connect the dots and know which apps are installed on a given device. But does my app's campaign benefit from that so much that I should go for the ATT popup?
Nope, they'll only know that some percentage of impressions of an ad lead to installs. With their SDK, they'll know who installed, and they can then feed this back into their ML models to find more similar people for you.