I'm talking about the compile-time type checker that ML and Haskell have. This is a very powerful tool that doesn't exist in dynamically-typed languages. It seems like a mere curiosity, because a lot of the super-cool things you can do with type-checking are impractical (e.g. the use of phantom type hackery to do bounds-checking on arrays). However, on large projects with multiple programmers, it also helps to keep interfaces stable and self-documenting.

For example, let's say you change the output type of a function deep in the trunk-- instead of returning a 'a or throwing an exception, it returns a 'a option, e.g. a discriminated union between None or Some x, where x is a 'a, using the None value to represent failure, in lieu of the exception-- and want to find all the places where things break in the leaf code, so you can adapt them to the changes. What tool do you use: grep? Nope. You attempt to compile the tree, and all the code-breaks are caught and displayed by the compiler.