What you say is, of course, true[1]. However, I find that the types actually do have a huge benefit, namely that you're "encouraged" to give more accurate types for your functions. For example: "frobnicate :: String -> String" has a huge state space, and if it actually really only works on a subset of String and you want to test it you have to manually provide "overrides" for the value generators... which is semi-painful if you have to do it for every test. So instead you add newtypes and change it to "frobnicate :: Blampf -> Schmeckle" and move all the "test-only" code into the Blampf and Schmeckle constructors[2] instead.

[1] I was actually about to footnote here that it was invented in Erlang-land, but apparently not: https://en.wikipedia.org/wiki/QuickCheck . Not sure why I thought that, maybe just because Joe Armstrong has been an advocate of QC-style testing for a long time? Or maybe it was he who invented QC in Haskell?

[2] Which, granted, in Haskell will mostly be run-time checked, but at least you'll get an error at the earliest possible opportunity.