No checking disk for errors will run disk utilities to see if there are problems with the disk where either repair is needed, or if sectors need to be marked as bad so that they end up not being used. It's the same as when you'd run
fsck(8) on a disk in your system.
You can run fsck on your operating disk, but it will only look at information whereas if you run it on a non-active disk, or partition, it will be able to repair problems. When the installer is running, the main disk you'd be eventually operating from is not active prior to the install, therefore checking the disk is a thing which can be done and usually should be done.
