As ASAP Science illustrates, the leading theory is that crying is an obvious expression of sincere sadness or distress, and that we're sending out a sort of biological bat signal that we're in need of comforting. And the fact that tears blur our vision which handicaps our ability to react shows others we're being sincere.
Another more interesting theory, however, is that the type of tears that flow when crying contain a high concentration of a chemical that's linked to stress. And since crying rids the body of that chemical, it could actually help us chill out when stressed.
Why Do Humans Cry?