Why Is It Unlawful To Drink And Drive?

If you drive after drinking you are more likely to harm yourself, harm someone else or both.  Alcohol affects the body in two main ways where driving is concerned.  It slows down the ability to make a sound judgement, so if a hazard appears in the road, it is more difficult for a drunk driver to avoid it.  
It also affects our ability to think clearly, so if an obstacle appears in the road, we are slower at being able to think what we can do.  
Combined, the lack of reasoning and the slow speed of response, means that an accident is much more likely to happen.  Even people who 'seem' sober after they have been drinking will find that these functions are affected by alcohol, even if they cannot feel the effects of drinking.
Driving whilst under the influence of drugs is also becoming a much more common offence and police recognise that drugs have a similar effect on a person's driving ability.