When you buy a rental house, it might include tenants in place, and these renters will suddenly turn into YOUR tenants. These tenants are called "inherited tenants.” Inherited tenants may be valuable, as you won't have to immediately spend some time filling the empty unit, and you are going to be getting income from day one. But inherited tenants may also be insecure, since they weren't set in place by you personally, and you do not have a very clear sign of just how well they have been screened or which kind of tenant they are.
What's more, they could have been badly trained by the previous landlord and will have to get re-trained to follow your principles. The exception to the rule is that those renters will be totally perfect, and you will be grateful to have them.