Reason # 1
If you don't neuter your cat, he may begin to spray in your house to mark his territory. Neutering will prevent this impulsive behaviour from beginning and it will often stop it from occurring if it is already present.
Sometimes cats will spray in your house (and outside, but I bet you won't mind that as much!!). Did you know that hormones play a big part in the spraying cat? If you neuter him, you may find that he doesn't spray as much.