Never belonged to a beach club here, but I grew up in a South Shore Long Island town that has perhaps 10 of them. Fun fact, the one right next door was where Donald Trump went as a kid; everyone in the area knew it as the one that doesn't accept Jews.

Most people join them because it's a scene. It's the summer version of a country club. There's usually a restaurant, there's a bar, you make a whole day of it and it's a lot more comfortable than just hauling your chair to the beach. You can go inside and kick back, go out to the beach, go inside and kick back again. Often they'll have chairs, umbrellas. They'll have kids working there to set everything up. You'll get a cabana so you can change inside, in privacy. There may be showers that are better than the ones you'll typically find at the beach, when you'll find showers there at all. There's often a pool too. You can get access to their facilities -- one girl I grew up with had her birthday party there every year. And then I'm sure more than a few people join them to make business and social connections.