As a landlord or property manager there are times that you think you've checked everything that you can. After you've preformed your due diligence, they look great on paper and you let them move in. Sometimes they turn out to be the fantastic tenant you expected, but sometimes they surprise you in the worst kind of ways. That's when you go back to your lease for help in handling the situation.
A lease will be made up of what is legally required in your state (including the specifics of how much is to be paid, when etc etc...) as well as the clauses that will be added to dictate what is expected of the landlord and what is expected of the tenant for the relationship to work. Never gloss over these clauses, because these are what you'll fall back on if you land yourself with an unexpectedly bad tenant. Did they bring in an un-screened house guest that is overstaying? Look back to your clauses. Did the house guest commit a crime on the premise? Again, look back at your clauses. You are providing your tenant with promises of what you will provide to them (such as quiet enjoyment of their rental and other such agreements to make sure that their stay in your unit is a happy one), so it is not out of the question for you to make some requirements as well.
Experience will teach you what clauses work best for you and which ones do not.
For further reading, check out BiggerPocket's article on favorite lease clauses.