Why does some state have laws that say you cant drive while intoxicated?

Answer 1

Actually, there's a federal law on this which applies to ALL states, and some states have opted to enact state laws which imposed stricter BAC limits.

The reason for this is simple. Alcohol is a depressant, and so are many other substances. When you use them, your judgment and reaction time are impaired. When you drive under the influence, you present a severe threat not only to yourself, but to anyone else on the road, as well. And, at this point, you might go into a rant about individual rights and such, but the right of the guy who works a swing shift on a Friday night to not get t-boned at an intersection because you were too drunk to react accordingly to the stop sign or signal outweighs that, greatly.