When bands/artists/performers play venues the sound is mixed for the accoustics of the room, to give the best possible sound for that gig. (The best sound is actually right in front of the mixing desk so stand there at the next gig!) If you were to record the sound out of the desk, it would sound flat, and incredibly poo. The way round it would be for line outs to be taken track by track from the desk, into another desk, Remix the sound and then record it.