Think about this: let’s say you run a program. Do you want that program to be able to take over the computer and read all your files from now on and send the data to a remote third party?
Probably not.
Permissions were created to stop programs from doing that. By running most software without admin permissions you limit the scope of the damage the software can cause. Software you trust even less should be run with even fewer permissions than a normal user account.
The system is imperfect though. A capability-based system is better. It allows the user to control which specific features of the operating system a running program is allowed to access. For example, a program may request access to location services in order to access your GPS coordinates. You can deny this to prevent the program from tracking you without otherwise preventing the software from running.
