Ranks are something that was implemented to make the game more challenging for you. You can buy ranks with gold, which you can earn by mining, Killing high rank players and by selling items in the shop. With higher ranks come perks and with perks come power. When you get to a higher ranks you will be able to use MCMMO skills like taming and swords to make yourself more powerfull. The usage of items like anvils, brewingstands, TNT and enchanting tables are also reserved for higher ranks. But there is a downside, Power comes with a price, and not just money. when ranking up to ranks higher than 3 you get a bounty on your head. So players will try to find you and hunt you down!
When you die your rank gets reduced by 3. This makes death something you don't want, People will play realy carefull which enhances the gameplay! Imagine a pvp battle when both players fight to keep their rank. Very intens!
There is a possibility to protect your base. You can yse a p-stone (cobbweb) to protect your base. A cobbwebb is a very basic protection. it will only protect against block breaks and block place. and NOT from TNT, Creepers, Chest opening..... and so on. Again there will ve consequences when you die. Your p-stone (cobbwebb) will be removed. Making your base raidable.