Well, it's not necessarily having a stronger will; you get that just by having a high Willpower.
But Higher Purpose/Power Hungry gives you a feeling of purpose, a meaning to your life, and it becomes difficult for any effect to alter your personality because you are focussed on that.
However- that can flip. Anakin had a Higher Purpose, and flipped it to Power Hungry. I had been making some small pushes with Roan in that direction as the Dark Side tempted him- not just the going nuts, but his thoughts about how peace had turned to violence and he had to start it, and the allegories with Kuylen.
It is this specific relation with a force user's personality that makes it unavailable in the OT game, where you can just take the higher Willpower.