A type system saves a lot of time when reading code that has been written by other people and has evolved over time. In Elixir, I often end up adding debugging statements to code and running it, just to check the data structures.

A type system also gives additional assurances when changing code that is used from many places. It's so nice to make a change and have additional confidence in it because the type system is happy with it.

I can't think of many bugs that I've seen that would have been prevented by a type system. I'd still like to have a type system though, it's sort of extra documentation within the code.