For one thing, it is a nice reminder that it takes a lot of work to make something look effortless. Bono once said of U2's concerts that they were trying to create intimacy on a grand scale. And with well-run shows, it seems that way. Somehow you feel a direct connection with the artists. You get lost in the moment and think that the concert just involves the few people on stage. But sound check is done with the amphitheater in full sunlight or in the arena with all of the lights on and the curtains pulled back. You see the buses, the trucks, and the large numbers of people who are needed to pull off the illusion of intimacy later.
The other benefit, from a social media perspective, is that sound checks are a treasure trove of content in the rough. You can get great, up-close photos and video of the instruments and the musicians. And while great photographers can capture amazing moments during the show, you get video and photos of a different flavor at sound check. Sometimes the musicians are more relaxed, quicker to smile. Sometimes they are more contemplative, focused on tuning, gear placement, etc. It is an interesting peek into who they are and how they prepare.
Sure, nothing beats the actual show. That time is magic. But for music geeks like myself, there is something special about watching a sound check. - Cris