UI is extremely important in gaming. UI provides the information needed to make decions in game. Common attributes of UI are health, stamina and currency. Seeing how much health you have lets you know how many more hits you can take. A stamina meter displays how much energy you have to perform special actions. Currency determines rather or not you can afford an item or service you're considering purchasing.
As you can see, UI elements are great for players who want important information on hand to make decisions. Some players have played games long enough to know what's going on without constant infomation being displayed. Those players might opt out of using UI elements. This increases immersion meaning the player may forget that they're playing a game.