Because if you don't, this is the sort of thing that can happen. Tinker Belle is a fourteen years old, seventeen pound Rat Terrier. They never got around to spaying her. She has had two breast tumors removed in the last three months, one malignant, and one pre-cancerous. Her owners asked if there was anything they could do to help prevent new ones. Since these tumors are hormonally influenced, removing the female hormones couldn't hurt. The googly thing in the picture is her reproductive tract, the uterus and ovaries. The dog's uterus is Y-shaped, two long tubes, joined near the cervix. You see the two "horns" spread out and leading to the ovaries. The swellings in the little tubes are filled with pus. Whoops! Good thing we got that out of there.
Here are the ovaries by themselves. The one on your right is the normal one, about the size of a black-eyed pea. SO... if the ovary on the right is normal, what does that make the ovary on the left? That's correct, it is abnormal, with quite the big cyst, which was probably cranking out some crazy hormones that Tinker Belle didn't really need so much of. She'll be a lot better off without that baby.
Getting rid of the pus-filled uterus was an unexpected bonus. This is the sort of situation that has been a low-grade debilitating problem. As she recovers from surgery we will soon find out just how bad that thing was making her feel. I predict she will have a quantum improvement in energy and attitude.
We are fortunate that we found this before it made her critically ill. A pyometra (pus-filled uterus) can spread through the bloodstream and poison the whole body. Now you're doing an emergency hysterectomy when your dog is old and sick.
This is Tinker Belle's fourth major surgery in four months. How much nicer it would have been for her, had she been spayed before her first heat cycle. No breast tumors, no female trouble, no catalog of operations in her old age.