Compute is dynamic. You might be above capacity for Christmas shopping, and below capacity at 4am in the middle of the weekend.
By varying pricing, you can be more efficient. People who can will smooth out that load. If I don't need to run something during peak hours, I might wait until off-peak. Google needs less capacity. Everyone comes out ahead.
For a profit-making project, dynamic pricing makes sense. I suggested free since the primary goal of co-lab isn't to make money (but they also don't want to subsidize it too much, so they do need to charge).