Players have limited visibility in the darkness and never quite know where you are going. In some games, darkness is good while in others it is bad. For example, in the Splinter Cell series, darkness is an ally, letting the player hide from enemies and sneak up on them. On the other hand, in Gears of War, staying in the dark will result in getting blown to bits by mutated bats.
In some games, the player is tasked with fighting through the darkness, like in Doom 3. Players sometimes have a flashlight that lights the way, cutting through the darkness.
Darkness is also frequently found in horror games, adding to the mood. There are times where the player is plunged into darkness; when the lights come back on, something appears in front of the player (for example Bioshock). Other times, darkness can serve as a type of invisible wall - for example, in a subway station, players might attempt to dive head first into the darkness only to realize that they aren't running anywhere.