All cats patrol a territory that they regard as rightfully theirs.  Un-neutered tom cats have a much larger area that they defend in an aggressive manner.  They are therefore likely to have more fights with other cats (hence exposure to Feline Leukaemia and FIV) and more likely to be involved in road accidents if their territories cross over busy roads.  They also urine spray to mark their territory and this often includes the lounge furniture!  Needless to say, we strongly advise routine neutering.