There are lots of private apis in iOS which developers are not allowed to use but would allow for a richer, wider variety of apps.

For example f.lux has a large feature set which isn’t built into Apple’s night shift. F.lux relies on a private api so the only way to get it on iOS is through Cydia or some other jailbreak App Store.

Also I can imagine there would be lots of possibilities for third party watches if apps could have properly backgrounded processes. Android watches are generally crippled on iOS when compared to the Apple Watch.