Don't want to get a stain

from here (image source)

You might be thinking this also stops people concerned about dirtying up the underside of their car, but if that was the only problem the driver could get out, go around and find the mechanism to open the gate. This literally only works against people who can't cross the grass. 

Now we know why they're so virus prone

from here

Computers running Microsoft operating systems (be they Windows or DOS) were not the only ones to get viruses, but viruses certainly were more prolific on them than any other kind of system. I wonder why that might be.