Here's a practical application of mfix:

    mfix $ \threadId -> forkIO $ do
      -- computation in forked thread

forkIO creates a new thread and returns its thread id. However, in order to access the returned thread id inside the forked computation, normally one would have to store it in a variable and then read from it inside the thread.

mfix, by nature of its laziness, captures the return value of the function passed to it, and passes it to said function.