There are many behavior and health reasons to spay (female) or neuter (male) your pet dog or cat. If you have no plans to breed your pet, it is highly recommended you fix him/her.
Aside from the obvious contributions it makes in controlling the pet population, all animals that are fixed have less inclination to explore the world (i.e. run away), as well as decreased aggression and dominance issues. This means less dog fights and cat spraying for all of us.