Because people will pay the cover charge. If you have a capacity for 350 people, you're open from 9pm to 5am, and people stay, on average, for about 4 hours, then you can reasonably assume that 600 people will show up for your club. Charge them each $20 and you just raked in an extra $12,000 for doing absolutely nothing. Why wouldn't you charge, especially when everybody else does it? The better your club, the hotter your clientele, the more you can charge. It's basically a service fee at that point.