well no you won't be using your phone any differently it will just feel and look better when scrolling. for most people this won't matter at all but I love it on my iPad Pro so would welcome this for sure.do they need it for marketing purposes? yes, almost every Android have it - and iPhone 'power' users want it
But does having 120hz allows regular joe to launch facebook faster, take better/faster photos, faster/more accurate typing NO - and yes, I switch from iPhone Pro to iPad pro and I don't any difference
if we get 120hz with ltpo battery life will be as good as who knows we may get AOD and under display touch ID...