For most, it's about making a difference and leaving the world a better place. Everyone now alive will have passed on. Your purpose has an ongoing and life-long theme. Your purpose taps into passions, inspiration, and commitment on a grand scale. Being able to know and communicate your purpose, to yourself and others, is what sustains it.
Vision is where the spark resides.
It is an image so compelling, so desirable, that it draws energy and motivation to it. It is Pinnochio as a little boy. It is men strolling on the moon. It is that oasis in the desert. It is once sickly children, now free of leukemia instead of dead.