When a function fails to produce the return value, it can either return a distinguished value (a NULL pointer, or -1 to signal an error, or ...) or raise an exception. The latter is sufficiently clumsy that it's not uncommon to see languages with two versions of some functions: one that raises an exception, and one that returns a distinguished value.

Haskell uses the Maybe type for the distinguished value, raising it to the type level. If I call a function that expects a value that is NOT the distinguished value (a valid pointer, or a valid array index, or ...) in languages without something like a Maybe type, I find out about it when my unit tests fail if I'm lucky, or I just  get the wrong answer if I'm not. When I call a function that expects an X with a value of type Maybe X in Haskell, it's a type error at compile time.