Originally Posted by zap
for some strange reason
You're installing graphics drivers that X depends on. If you change the drivers that it's currently using, some things may stop working (OpenGL apps won't start, at least; I've heard reports of X crashing on some systems). Furthermore, if you upgrade the kernel module the old one can't be unloaded -- and the new one can't be tested -- while X is running. So even if you install while inside X and restart X, unless you do `rmmod nvidia` while X is not running it won't work. Or, if for some reason the module doesn't load properly (unresolved symbols or such), then X will just fail to start, and you need to fix the installation while X is not running (as opposed to the installation simply failing without touching your configuration, so you can start X, research/fix things and try again later).