If the rear tyres loose grip when you are doing an stop, the back end of the car will end up in front with you looking where you have been and not where you are going. Not good.
Under normal conditions and even some harsh conditions you will not get that problem, but it may show up under emergency conditions, which of course is the worse possible time.
If you look at the tyre manufacturer, car manufacturer and tyre retailer sites you will find they all tell you to put the best on the back.