Although as mentioned before the pee-in-a-cup part is optional, it saved a friend's ass. That allowed for detection that something was wrong, which prompted him to do more tests, and resulted in a diagnosis of a severe white-cell blood problem. Save for this check he'd be dead by now.

It may seem mundane when everything's OK, but thanks to this check many people discover they have diabetes, kidney issues, hearing or sight issues (both are tested at regular intervals) and can then take proactive actions to fix that, usually suggested during the last part which is the debrief with a doctor†. Similarly, that chat may look useless to healthy people but sometimes it help uncover psychopathological situations such as nascent burnout or depression.

Seriously, what's there to be so up in arms about it? It's all handled confidentially like every medical checkup, the employer can't see anything of it except a mention saying you're medically able or not able to do your job, it helps prevent drunkards driving school buses, and it may save or at the very least improve your life.

† Caveat: Some doctors do really suck and could care less. Sadly happens, but not all of them are bad.