They tell you if you broke a class of things, and I think that class of issues is pretty broad: * when you remove a method from a class, no user of that class

They tell you if you broke a class of things, and I think that class of issues is pretty broad: * when you remove a method from a class, no user of that class calls that method * when you change the return type of a method, all users of that method are updated to handling the new type * in type systems with nullability, they make sure all users handle null values, and they make sure methods with non-null return types never return a null value There are functional changes which can break your code which won't be caught by the type system, but there are a lot of details which the type system will make sure you check when you make changes.