If you stand in front of a bunch of Marshall stacks loud enough to fill a stadium with sound, your hearing would be gone in about an hour. PERMANENTLY!

The PA system is set up like a very large version of a home stereo, with all of the instruments, drums, and vocals, mixed together in balance. The speakers are usually off to the sides of the stage, and elevated above the crowd, so people near the front don't get totally deafened. This also allows the sound to be projected in a controlled manner, so that people near the back can hear it too.