Not every piece of logic lends itself to being expressed in the type system.

Let's say you're implementing a sorting algorithm. After step X you can be certain that the values at locations A, B, and C are sorted such that A <= B <= C. You can be certain of that because you read the algorithm in a prestigious journal, or better, you read it in Knuth and you know someone else would have caught the bug if it was there. You're a diligent reader and you've convinced yourself of its correctness, working through it with pencil and paper. Still, even Knuth has bugs and perhaps you made a mistake in your implementation. It's nice to add an assertion that at the very least reminds readers of the invariant.

Perhaps some Haskeller will pipe up and tell me that any type system worth using can comfortably describe this PartiallySortedList<A, B, C>. But most people have to use systems where encoding that in the type system would, at best, make the code significantly less expressive.