The area where the chimney comes thru the roof is called the flashing.
Any water that enters thru this area will find its way down the chimney and get between the wall boards of your living area. You will not see the water until it works its way thru the walls and exposes itself in the living area. In the meantime mold can build up between the walls causing health problems.
We do not recommend using Tar because in the winter time this tar will become hard and as your chimney freezes and thaws this tar will crack because it is brittle. In the summer the tar will sag or drip and open these holes back up.
We use a product that expands with your chimney and the roof as it expands creating a good seal.