Better Hardware Support
Installing Linux often comes with a few hours of setting up the PC's hardware. On a pre-installed PC, all hardware should work out of the box, wireless, screen resolution, blue tooth, etc. These should all have been tweaked and tested by the vendor. If there are any issues with hardware, a reputable vendor should point this out before the sale. While the newer versions of Linux do support many types of hardware, a computer pre-installed with Linux should arrive ready to go, without the need to install additional drivers or kernel modules.