It's used for if you need to reset for a wanted thing to happen that is random in a short amount of time as it gets rid of the time needed to turn on and off your game. Instead, the only time thats taken up is the transition to the title screen. So it is a way to save time.
For example, if you want a specific nature for a stationary Pokemon like virizion in B2/W2, you can keep reseting until you find one with the nature you want.
It is also used if you are shiny hunting for a stationary Pokemon. So say I want a shiny latios in B2, I could keep soft-reseting until I find a shiny one and then I can catch it.
Also it does not delete your entire saved data. It only deletes what youve done after your last save.