Sometimes I take a legally prescribed stimulant / controlled substance for ADHD. Those medicines can be perfectly safe to drive with once you know how the particular dosage affects you, and driving with them is often even safer at the appropriate therapeutic dosage than driving without them. Further, as a person with ADHD and a tic disorder, I would have a fair chance of failing a field sobriety test even if I'm sober. I'm also not thrilled with the idea of lying to cops, since I know that can be a crime separately from the question of DUI.
So, putting all that together, imagine this sequence: cop at a DUI checkpoint asks me to perform a field sobriety test. I refuse, either without giving a reason or citing my ADHD and tic disorder. They ask me if I'm taking any medicine for ADHD. I don't lie and either confirm that fact or plead the fifth, or even if I do lie they still might not believe me. I probably don't have the pills or the bottle with me since I wouldn't usually be taking it in the car anyway. They then insist on a blood test, either with consent or with a quickly obtained warrant. I then have to accept a long detour going for the blood test, and then spend a lot of time and money proving in court both that I had a legal prescription and that I was not legally impaired by the medicine. (Even if I do have the correctly labeled pill bottle with me in the car, the cop still might incorrectly assume the medicine impairs me.)
Avoiding this hassle is a perfectly legal and legitimate reason to want to know where DUI checkpoints are.
Nothing I'm saying is condoning driving drunk - I certainly don't do that. When I drink, I pay attention to the advice of blood alcohol content calculators to figure out when I'm safe to drive and when it's fully out of my system. And when I take medicines that interact with alcohol, I'm even more cautious with drinking than when I don't.