Because of types, basically. You can construct the core data structures of your program and the types of the functions operating on those data structures without having to actually implement said functions. When the types fit together, i.e. your program is logically consistent, you can start filling in the actual code.

This is much faster than writing a large chunk of code, playing around in the REPL thinking everything is ok, only to realize one week later that you didn't think through some core aspect of your design and then having to refactor a sizeable piece of your codebase.

> The last message I get was something like "Array.Array: Index out of bounds". Program crashed. No idea where the error originated.

It is an unfortunate historical artefact that there are still partial functions (i.e. functions that may crash) in the Haskell base libraries. It is generally advised to avoid such functions, but this is not very clearly communicated to newcomers to Haskell.

For example, Elm, a language largely inspired by Haskell, doesn't have partial functions. This is why it prides itself in "not having any runtime errors in production" (excluding things like compiler bugs or unsafe JavaScript libraries, of course).