A recent surge in popularity has brought different strongman training methods into mainstream facilities. While this is largely a good thing, it also requires an understanding of what the heck these exercises actually do.
McGill (2009) showed that the lats are very important for a lot of the carrying events such as the farmers walk due to their role in stabilizing and compressing the spine for additional support as well as providing a co-activation potentiating effect for the prime movers.
A fist is way stronger than any of the individual fingers, so getting the muscles to all work together helps produce stronger contractions. The two-barbell variation really lights up the lats for this reason.