The scheme stuff was added because Xcode became able to do a lot more stuff beyond "Run app" and "Debug app". There were a lot of hoops you had to jump through

The scheme stuff was added because Xcode became able to do a lot more stuff beyond "Run app" and "Debug app". There were a lot of hoops you had to jump through to get unit tests working and there weren't iOS apps that could be run in a simulator or device.