The purpose is to give the teams an incentive after the game has been 'decided'. You are losing by 14, there's 1 minute left, you might think "**** off'. But if there is a point to be gained by scoring a try, you might go all in.
I am 100% in favour of the point difference BP.
I am 100% against the number of tries one. I would be ok if instead of tries the difference would be measured in points. I find the point discrimination aspect of this ridiculous. Want to make tries more appealing than kicks? Then award them more points, ffs. It's not rocket science.