If you want to do real-world things, like change your project settings across different compilation targets (AppCode has a buggy/incomplete settings interface that can't be trusted), use the interface builder (inc. previewing your UI, inc. creating/editing the UI if you're not using SwiftUI), debugging the UI hierarchy, performance profiling, etc., you'll be switching over to Xcode. Once again, in theory you could just have everything running off whatever `make` toolchain you prefer, but if you're making apps for a company, and sharing code with other team members, chances are your exotic setup isn't going to cut it.

i think people might be conflating needing to install xcode vs launching xcode

if you want to build ios apps installing xcode is a requirement (to get the proprietary cli tools) but launching xcode is not required

to build native mac apps, neither installing nor running xcode is necessary, as you can build with the smaller cli-only tools which include only mac os headers/libs