Why do cats hiss at each other?

According to Animal Planet, cats hiss at one another when they feel threatened. For example, when a new cat is introduced into a home with another cat, the resident cat may hiss to show the new cat that it is on its territory and to compel it to back off.

According to Cat Behavior Associates, cats do not like physical confrontation, and hissing is a way to give off a warning to another cat or animal without engaging in physical combat. The Nest recommends introducing new cats into a household slowly to help prevent both new and resident cats from feeling threatened or scared.