Short version: Most, if not all programming bugs are due to your code receiving something it did not expect. Haskell's type system helps eliminate this problem by forcing your code to declare exactly what it expects, and to handle all cases of said expectations.

In other words, it's doesn't let you skip out by not thinking about the entirety of your code. It's the same line of benefits that TDD gives you, but TDD is, IMO, a stepping stone towards the pure, functional programming that Haskell provides.

Note that I am by no means a Haskell programmer. To me, Haskell is the "last level" in learning development. I hope to one day reach such a level.