Cats can actually become pregnant while nursing their young. Male cats tend to reduce territorial spraying, depending upon their age at neutering. Neutered dogs and cats fight less, resulting in fewer battle scars, contagious diseases and abscesses. Males also wander less because they aren't as interested in pursuing females in heat. Also free of the urge to mate, dogs and cats tend to be more content and calmer.
Neutered pets are easier to live with.
However, guardians over-feeding them, and not providing enough exercise usually causes weight gain. In fact, an unaltered male can be responsible for impregnating dozens of females. Most vets will spay female cats while they are in heat, and some will spay female dogs in heat. The cost is generally less than most major surgeries, such as the vet care required to repair animal fight injuries, or preventable cancers. This is especially true of young healthy animals.