It's not just for existing. Pets can and do cause a lot of damage. They can urinate on the floors or walls, leaving odor that is very difficult to remove. They can chew up or scratch up doors, walls, floors. A pet can easily do thousands of dollars in damage in a one-year lease term. Of course not every pet does this but you don't know in advance.

On the other hand landlords compete for tenants. So they may want to allow pets but impose a surcharge for the potential damage.