I can give several reasons to re-install Windoze.
When the person your helping insists on perusing porn sites and the stuff they
get there kills the machine to the point that it won't boot.

when another person decides to just deleat a program or 2 or 3 not un-install
and they deleat the wrong stuff.

when someone plays in the registry without backing it up.

And most recently some one who jumps from WIN98 to XP and is a die hard DOS
military man and thinks he should be able to force the new machine to play WIN95
and DOS games. His old 98machine died and I built him A new machine of course he
wanted 98 but the hardware didn't. Have fixed it twice and just re-installed. I
told him he breaks it again he's on his own.