Lots of little quality of life things. For instance, in Kotlin types can be marked nullable or not. When you are passing a potential null into a non-nullable argument, the compiler raises an error.

But if you had already checked earlier in scope whether or not the value was null, the compiler remembers that the value is guaranteed not to be null and won't blow up.

Same for other typechecks. Once you have asserted that a value is a given type, you don't need to cast it everywhere else. The compiler will remember.