Airport lounges are typically for people who hold status with that particular airline. Take United for example. If you are a Mileage Plus Member, you can purchase the access to the Red Carpet Club. The amount of the fee is based on your status with the airline. You can pay for it with money, or with miles. When you get the access, it's good for 1 year. It's not something you would probably do if you only fly a couple times a year. It's more worth it for people who fly all the time.