It's not a stretch to say the App Store changed the way people purchased apps for mobile devices. Back before the iPhone, mobile devices, such as those running Windows Mobile (and its many predecessors) and Palm OS went directly to developer sites to download installers that were then used to load apps onto devices. It was a mess, updates were tricky and there was no real oversight that the apps were safe. The App Store changed that. But it's also a closed environment that Apple has fought to maintain. That control is being debated in the US Supreme Court and Apple seems to have a fight on its hands.
Apple's argument, which is covered in great detail here, boils down to saying you and I aren't Apple's customers. It says we are the developers' customer and therefore a class action by App Store users isn't legally possible. Also, Apple contends that the 30% commission paid on apps distributed through its store, which is the same commission charged by Google at its Play Store, isn't paid by the customer but by the developer to Apple as a fee for services Apps provides with distribution and managing payments.
At the heart of the matter is the question of whether Apple's customers should be allowed to side load apps easily and whether rival app stores should be allowed to operate, as they do for Android.
Interestingly, the popular game Fortnite is distributing its APK directly and bypassing the Play Store to avoid the commission and hang on to a higher margin.
Apple will continue to fight for control of the iOS app environment. Even if the Supreme Court rules against them, the current case is only about whether an antitrust lawsuit can proceed. If the Supreme Court says the case can continue, it will be tossed back to a lower court for decision and, I assume, an almost endless run of appeals.
Although I am an iOS user, I think this is one case where Apple ought to back down. Apple can maintain their App Store and tout its benefits over rival stores and side-loading. The company can maintain some control over the total operating environment through the APIs it makes available to developers and ensuring the security controls it has in place whenever an app tries to access a sensitive service or set of data, such as location, photos or contacts, are robust and protect users.