First and foremost, we are hard wired to participate in sports where we accumulate points. It is in our DNA and it is a huge motivating factor. Getting and counting points is fun. Second, the current stroke based scoring system does not equitably reward a player for making a good score. It makes absolutely no sense that a birdie (which, for some of us is rarer than a solar eclipse) is worth only one stroke less than a par.
The GPI treats each hole as a stand alone challenge and rewards you based on how you do in relationship to par. A weighted scoring system also allows a player to make up for a blow up hole (or two) early or late in the round. Currently, it is virtually impossible for a casual amateur player to make up for a double or triple bogey. The GPI allows you to make up for a 0 on one hole with a 3 or even a 5 on the next. It keeps you considerably more focused and engaged- especially if you have had a bad start. Conversely, a bad finish does not blow your entire round because you have already booked your early points which can not be taken away from you.