The environments in games are one of the main key elements, it is the world you see and play through, and they have great impact on the gameplay. Not only should the environment be affecting the atmosphere of the game, but it also plays a big part in the gameplay itself. For instance, a large open world which you can explore freely will be the right sort of environment for an RPG or a sandbox style game. On the other hand, a realistic FPS game will have closed in environments, with buildings and streets where you can hide and take cover etc.
Generally, in levels, the designers construct them to lead you through the level. They are built in ways that perhaps are realistic and recognisable, so that you would know where to go, or at least be able to figure out where to go. They have to be built and decorated in some form of logical way, so that you can understand the environment and be able to travel through it and explore.