Citizen Linus

No idea (other than the number of keypress "beeps" from the machine is then the same as a valid vote). I hear from time to time rumors that it makes a difference when counting the votes, but never saw any reliable information about whether its true or which difference it makes.

I think it was made that way because back when you had paper ballots you could either leave it blank or invalidate it (marking multiple candidates, for instance), and they wanted to keep the same possibilities on the electronic voting machines.