what can you run on ARM that you can’t run on x86?

Run arm apps natively?

What apps? Name one.

Doing development for ARM is easier when you have an actual ARM machine. Any project with assembly in particular.

I find pushing builds into an image and running them under qemu-system-aarch64 far easier than screwing around with hardware.

Not the parent but in developing and testing server software it's necessary to have the target architecture available because all sorts of random crap changes between architecture to the point that once things become more than trivially complex, the probability if things breaking when the CPU type is changes becomes close to 1.0.