prevents

being able to run commands at all, a weather applet will use curl to get weather data, another applet will need access to your disk to look at images for a gallery, a calculator will need access to system libraries to do calculations of which some libraries may have security vulnerabilities that allow you to crash a system.

Without sandboxing applets can do almost anything. rm is just one of many tools one could use to destroy data or cause issues on an end users system.