The main reason to spay or neuter your pet is to reduce the number of unwanted pets in an already oversaturated population. According to the ASPCA there are several medical and behavioral benefits as well. https://www.aspca.org/pet-care/general-pet-care/spayneuter-your-pet
Here's the top ten list from the WebMD pets section of why you should spay/neuter your pets:
So please help us help them, we'd like to see a lot less unwanted, unloved pets in the world.