In game interface

There is allot of detail that goes into in game interface that is more commonalty known as the HUD (heads up display) this includes the mini map, team symbol, score and crosshair.
The in game interface is a large part of game because if you get lost then you can wonder your eyes to the mini map and find the right way or if you have lost an enemy. The other help that an ingame interface is that when you run in to a firefight then you can keep an eye on your health and ammo count also on some games e.g. Borderlands the enemy's life level. If there was no in game interface games developers would have an almighty struggle to get custom because the gamer wouldn't be prompted when there is an door to open or a weapon to pick up and this would  make that games developer loss there sales then even worse their company if their game is that bad to play.