Your Early Impetus is the original idea or motivation that inspired you to tell this story in the first place. For example, in Soar, our Early Impetus was that we wanted to find a single, remarkable person with an inspiring perspective to include in our upcoming web-series, The Remarkable Ones; and we were particularly looking for someone in Australia, since Patrick would have a few extra days while he was there.
Your Purpose, then, is much more developed and intentional than that.
Your Purpose is what you want to leave your audience with, as developed through your 5 Keywords. So in Soar we ultimately wanted to convey our Keywords for Dave—Possible, Mind, Appreciation, Able, and Soar—and leave the audience with Dave's greater truth, that you're only limited by what you think you are.