Burgers are the server's second currency. Ranks add slots, never combat or mining power. Everything here can also be bought in game with /burger using Burgers you pay for with in-game $, so the store is a shortcut and never a requirement.
Ranks only add slots and a chat prefix: more homes, more auction slots and more order slots. Each rank lasts 30 days and renews monthly until you stop it.
You can also get Burgers by playing: sell items with /sell, then buy Burgers with that money in /burger (10 Burgers = $50,000, 50 = $250,000, 100 = $500,000). The store is the quick way to the same Burgers.