Meta makes the Facebook app. Currently, the only way to get the Facebook app on iOS is through Apple's App Store. Which means Meta has to follow all of Apple's guidelines.
Once sideloading is allowed, Meta can make a version of their app that does not follow Apple's guidelines.
Now, they can maintain two apps, but last time I checked, the cost of maintaining one app was lower than the cost of maintaining two.
Eventually, the App Store version will no longer work with Facebook's API. Or the iOS version, or a third thing. There will only be the sideloaded option.
I don't have the freedom to choose the version of the app I want. And now I can't actually trust any of the apps out there because none of them are required to follow any of Apple's guidelines.