Neither processing time as a statistic, nor the results returned can be fairly compared. Therefore 'Points, what are they fore ', aka credits were created to allow the measurement of the relative work effort done of one members computer versus all the others. For instance, the following:

P3 1,000mhz, 2 hours = 80 points
X2 2,500mhz, 2 hours = 200 points
C2D 3,000mhz, 2 hours = 240 points (for 1 core)

Very simplified, not considering factors like ram size, network speed, hard disk storage allocation (only a factor for UD agent), a C2D computer does in 2 hours, 3 times as much work as a P3.

Other than allowing individual and team comparison on basis of points, there is no payback. WCG donates for free their servers and technical expertise, the members donate SPARE CPU TIME and the results are provided for free to medical science. No money changes hands.