I guess the idea is that negligent driving isn't always obvious either to people watching or to people doing it, since most impaired drivers don't cause accidents on most of their trips (and many won't visibly weave back and forth). If some kinds of impairment significantly increase the risk of an accident but aren't easy to detect from behavior or from introspection, it might be a big benefit to safety to say that you can never drive a vehicle under those conditions.

And I think the introspection part could be important alongside the enforcement part, again because the idea is that people might routinely be overconfident in their ability to drive safely after consuming intoxicants. So they don't necessarily think they're endangering others as much as they are.

(That might be possible to measure empirically, and it might turn out that some existing laws are too strict under that rationale.)

I remember a children's museum exhibit that was about reaction times, basically trying to justify to kids why they shouldn't drink and drive when they got older. The interesting issue that it highlighted was that fast reaction times can be very important for avoiding some kinds of accidents, and even a small impairment to your reaction time could increase risk a lot in a way that you might not anticipate. But I also remember that when I saw that exhibit, I thought "hmmmm, why don't driving tests test someone's baseline reaction time to make sure that everybody who's a licensed driver can actually react to stuff quickly?" and "hmmmm, why don't they make a toy that you could use to test your own level of impairment before driving, rather than only relying on estimating your blood alcohol content?".

Edit: someone already suggested both of these ideas in another thread above!