On a personal computer, they "can't do much" to the things you can trivially re-create by reinstalling anyway. Apps, system files, etc.
They can however do everything to your own files, steal your documents, bank account data, and more.
That a progran run as you without root "can't do much" made sense for multi-user Unix services, not for a personal computer and your own files.
>Wonder why the most important systems in the world and big tech's servers run GNU/Linux? There's a reason
Yes, and it's not because "unless you log in as root, the apps can't do much" on your personal laptop.