When there are neighborhood cats around that nobody owns or takes responsibility for, it can lead to a ton of kittens quickly. Spaying and neutering cats is one way to avoid this, but it's hardly where the benefits stop for stray cats. If you've noticed cats in your area that are in heat, visibly pregnant, or who appear to be young females, then you should strongly consider trapping them, having them spayed, and then either adopting them out or returning them to the neighborhood depending on their friendliness.