When we don't spay or neuter our cat, we create a situation where there are more cats without homes.
Even if we give those cats homes. These are slots other cats desperately need, aren't they?
We don't do our cats any favors by not altering them, either. Driven by their hormones, they will howl, escape from the house, roam and fight, and pick up injuries and disease. The "call of the wild" is Nature making sure the next generation of cats gets born. That's going far too well, to judge from the number of needy faces in the shelter.
This is what creates feral cat colonies, where homeless cats draw on their instincts to survive; not in the actual wild, but in our civilization. They are seen as nuisances and usually are either harassed and neglected, or taken to shelters where they are killed as un-adoptable.
This is a tremendous amount of suffering and death that can be completely avoided; simply by making sure our cat starts out life as a beloved pet, and getting the altering that will keep them in this happy state.
For more about all the effort that goes into saving one such neglected cat, see Tristan's Brother?