The test does what is says it does, it measures your response to 75g of glucose, which is obviously so fast acting, the spike, how quick, when and how high, the hormonal (insulin) response, and the usual one (two hours) to see how the second phase does at that second hour.
An endocrinologist can glean a lot of information from it.
An extended oral glucose tolerance test (5 hours) can be used to see other criteria of how insulin/glucose imbalance happens after two hours. For many reasons the BG levels after two hours may not be close to where a non diabetic would be.
Insulin resistance, and weak first phase insulin response is highlighted.
Hence, the gliptin!
As I said, this could reduce the spike, as it did for me, from over 12(ish) to 8.4(ish) from 75g glucose.
I have very weak first phase insulin response. So initially it helped. But if I kept to keto, I would have no spike anyway!
Doctors have shied away from the test, due to cost etc. And rely on hba1c. Which doesn't show the details of how your body reacts to carbs and sugars.
Hope that helps.