If neutering or spaying an animal is necessary to prevent unwanted babies, why not just give them a hysterectomy or vasectomy instead? Do the remaining hormones still have other negative effects?
Removing testicles is easier/cheaper than a vasectomy, and has the added effect of making males less aggressive and territorial. Steers and geldings are easier to handle than bulls and stallions, neutered dogs and cats get in fewer fights and have less urge to roam.
As far as females go, removing the ovaries is said to have some reduction in risk of cancer (can’t get ovarian cancer if you don’t have ovaries), but I’ve never noticed any positive behavior changes. I’ve only ever had female cats altered, and I asked for just the hysterectomy.
Spaying and neutering have the added benefit of reducing or preventing several kinds of cancers too. If the testicles and ovaries are removed, they can’t develop tumors. Unspayed female dogs have a 25% chance to develop mammary tumors, compared to 0.5% rate for spayed dogs and 12% rate of breast cancer for humans.
Spaying is a hysterectomy plus some.
Neutering can be notably quicker than a vasectomy. In a vasectomy you cut open the sack, snip the tubes, and tie them off. In neutering you cut it open and rip them out. Significantly faster when doing it on a large scale with things like sheep. The other option is banding but that is quite painful for the animal for a long time.
There are also significant behavioral differences between a neutered male and an unneutered male. Horses aren’t neutered for population control, they’re neutered because Stallions are assholes and Geldings(neutered horses) aren’t. Same goes for Bulls vs Steer.
There isn’t as much of a behavior difference between spayed and unspayed animals that aren’t prone to getting knocked up at random, so spaying is almost never done on large animals where it is a much more extensive procedure like horses
Spaying a female animal IS a hysterectomy – the uterus is removed.
Yes, the hormones are a reason for neutering male animals. Testosterone can cause aggressive, territorial behaviour such as fighting and spray-marking, or wandering in search of females.