Your pet will be healthier. Neutered pets are less likely to roam out into traffic or get injured in fights. They have twice the life expectancy and are less likely to develop breast, uterine, prostate, or testicular cancers. You won't have to deal with staining or spraying. Unspayed female dogs and cats go through heat periods and experience a flow of blood.
Unneutered males mark their territory with strong smelling urine.
Altering pets eliminates these problems. You won't have to fight off suitors. When a female dog or cat goes into heat, males for blocks will know it. When walking your female dog you won't have to deal with males following her. You won't have to find homes for the offspring.