Type annotations on functions in Haskell (or similar languages) are useful for:

1. leveraging the type checker to verify (aspects of) the correctness of your function, and

2. communicating intent to humans

I've found in my own explorations with Haskell that its useful to write with functions with them, then verify that they work, and then remove them to see what the inferred would be (since it already compiled with the annotation, the inferred type will either be identical to or more general than the previously declared type), and generally (because it is good practice to have a declared type), replace the old declared type with the inferred type (though sometimes at this point also changing the name.)