The iOS Simulator consists of (some of) the userspace components of iOS compiled to run on macOS. This would be a full iPhone image running as-is via virtualization.

Here are some reasons to use this instead of the simulator: security research, testing, automation

Looks like this tool can provide a jailbroken environment for the latest iOS versions whereas one does not exist on iOS simulator or a real device

The simulator is a completely different OS. It may even still be using Intel.

I always figured that Apple never set up a true emulator (like what Android does), because they didn't want people exploring their OS with a debugger.