> Where is the harm in getting data on how your own body deals with glucose?
We have a relatively good indicator of long-term glucose levels: HbA1c blood testing. It's included in a lot of physicals now because it's relatively cheap. It's not 100% sensitive to every possible condition, but it's quite good as a screening mechanism for the general population.
CGMs will often give an "estimated HbA1c" value based on statistics from the collected data.
The challenge with CGMs is that it can he harder to know what's "normal" or not than you might think. There are a lot of stories of people becoming unnecessarily worried about occasional spikes or dips that are virtually inconsequential in the grand scheme of things.