Neutering will most likely eliminate spraying or marking done by males, but if the habit is established it may not. Spaying will reduce the risk of breast cancer. The chances of a spayed female dog getting breast cancer is greatly reduced. Cats are less likely to have breast tumors, but if they do have them they are almost always malignant. Spaying almost eliminates this risk.
Uterine disease is no longer a problem after spaying.
Ovarian systs that can be sometimes very painful are no longer even a consideration after spaying. Neutering significantly lowers the risk of prostate cancer in male dogs. Neutering eliminates the risk of testicular tumors as well. Spaying and neutering also prolongs a pets life. Almost by twice the life span in cats, and a significant number of years for dogs.