Chess, that's the whole idea: the majority of the sound in a hall reaches the listeners by way of reflected reverberations from various directions rather than direct sound from the front. Extracting this ambience material from the front channels in 2-channel sources, where it necessarily had to be mixed(there was no place else to put it, of course)and sending it to the surrounds where it belongs makes the listening experience resemble the original a little more closely. This is supposed to be a relatively subtle effect, and the surrounds shouldn't be played so loudly as to draw attention to themselves.
