It's not all about targeting, it's as much about attribution.
If you were the developer of an ad supported app (or website) and people paid you because their ads appeared and people tapped/clicked them your income is reliant on being able to measure that people do indeed take action on the ads. Otherwise advertisers wouldn't pay you. If you can't measure it your income is gone. This is why Facebook is suggestion it's 'audience network' product is dead (basically ad placements in apps/websites)
The IDFA - in particular for advertising in apps, allowed advertisers and app owners to be able to measure what happened and agree payment. Or if your trying to grow an app, and run ads to encourage people to install it - you'll have one arm tied behind your back trying to work out which ad networks are actually resulting in people installing your app.
The net result of this could actually perversely create the opposite outcome Apple are trying to achieve. Guidance from Facebook has been for
app developers to install the Facebook SDK into their apps to help it measure ad conversion (even if you have no other use for the Facebook SDK. Google have done the same - install their 'firebase' analytics platform in your Google and iOS ads to help measure conversions and ad performance.
This means facebook and google will get data from every tap you make in these apps, rather than just knowing you installed it.