This difficulty basically turns the game into a full-on survival experience, with real stakes and no manual or autosave. However, Bethesda didn't want to just stop there, which is why Survival difficulty is an official feature in Fallout 4.
From being chased by Deathclaws or getting mauled by ghouls, there's already quite a bit of challenge within the basic, vanilla version of the game itself. The world of Fallout 4 is a pretty grueling one.
