The Site store is where you can purchase ranks, permissions, and other stuff with your "Cash" (not real money, can be found in your "Statistics") The store's purpose, to make other users know that you have gotten cash and spent it on something which makes your account look better, making them want to get a custom rank, or something. So they try to save up there "cash" making them more active.

How to get "Cash":
You can get cash by being a very active user, (AKA posting alot (and none spam)) by recruiting new members to the website (you get more more cash if the recruited user posts) and I can check your IP so don't make a fake account and post under it. Also you can get Cash by contributing real money to the site. Check the "Contributation" thread why you would do that.