Apple's subjective quality standards give it broad discretion to reject apps from competitors or new entrants, and the determination of what constitutes sufficient 'functionality' is made unilaterally by Apple's review team.
Apple's App Review Guidelines directly affect what apps consumers can access, what data those apps may collect, and how they can pay for digital content — Apple's mandatory in-app purchase requirement means app developers cannot offer cheaper prices through external payment links on iOS. Consumers benefit from Apple's content and privacy review standards, which prohibit apps from collecting data beyond declared purposes and require age-gating for adult content, but have no direct appeal mechanism if Apple removes an app they rely upon. You can review an app's declared data practices by checking the App Privacy section on any App Store listing before downloading.