Modal logic is based on the logic of statements by adding new signs that allow you to Express the relationship of certain statements to the surrounding reality. The purpose of various systems of modal logic is to include in logic so-called modalities - first of all, necessities and possibilities: what "should be" and what "might be".
Classical logic deals with statements that assert the presence or absence of a particular situation. However, in life you have to deal with statements that contain an indication of necessity.