It's to measure insulin response in diabetic people. Also it's not plain, it's pumpkin spice! :p

The goal is to give someone under monitoring a very large volume of sugar in a controlled environment and measure their body's insulin response. This is particularly a test of Phase I insulin response when your body responds to sugar intake by releasing stored insulin as opposed to Phase II insulin response where your body secretes insulin as it is produced to bring down elevated blood sugar after the Phase I stores are depleted.

That said this is a nutty amount of sugar and I had no idea that they made them this large.