A pitot cover will help prevent water getting into the pitot head, particularly driving rain from a storm.
Err.....no, not necessarily. I have had water in the pitot system after heavy rain overnight and it had been protected with a pitot cover.
Yes, it will work for most singles where the pitot tube is under the wing but with twins and the pitot mounted under the nose, the rain runs down the nose fairing and straight into the pitot cover.
It makes it a good idea to check that the airspeed is alive at the start of the takeoff.
