What is modal logic for?

Question

I understand "pure" logic as a structural description of what a valid proof is but I have never understood the reasons for using modal logic. What's an example typical of how modal logic is used?

Answer 1

Modal logic can be considered an extension of or complement to common formal logic structures. The latter typically deal with strict truth values and predicate-subject relationships (e.g., all dogs are animals). Modal logic -- and there are many forms -- deals with different kinds of relationships such necessary or possible, as ought or should, probable or improbable, allowed or forbidden, etc.

Modal logic remains controversial. A good (and entertaining) introduction and summary can be found here, along with examples:

https://www.coopertoons.com/education/modallogic/modallogic.html