I'm single with two cats so I really don't need Costco, Sam's or BJ's but for many things it makes sense.
Examples are the bulk nuts like cashews,
buying cakes for work parties,
buying 1 huge chunk of Dubliner Cheese for $10 when I can get an itsy bitsy piece at Publix about 1/5 the size of the Costco brick for $6.
buying my eyeglasses there way cheaper than anywhere else,
vitamins in humongous sizes for cheap prices and good quality,
liquor store, photo processing or making picture discs
and many more things that are good deals.
They also give you cash back if you get the upgraded card and use Amex. I don't have either but if you buy alot I guess it makes sense.
You gotta be in good shape to shop at these warehouses cause the boxes sometimes weigh as much as me and can be kind of tricky getting them into the car and out when I get home, especially the humongous kitty litter which weighs 40 pounds.