Videogame players have a tendency to experiment. It can be really fun to mess around and see what the game allows, and how it reacts when you try something really different or unusual. Creating results for the player's experimentation is a way of rewarding those players who enjoy messing around like this. So by filling the game with details and creating results for off-the-wall player actions, a game developer can both make the world of the game feel more real and add in an additional fun-factor.
Hideo Kojima, the producer of Zone of the Enders, knows this. One big part of his style of game design is to put a lot of easter eggs and responses to random player actions into his game. Metal Gear Solid 3 has some of the best examples of this. Most notably, it is possible to completely circumvent a major boss battle against The End by utilizing a very short window of opportunity to snipe him immediately after a cut-scene earlier in the game. In addition, there are dozens of ways to mess with regular soldiers, allied characters, and bosses. You can tranquilize an ally character and then listen to her mumble in her sleep. You can play with a boss's head by putting on a mask that looks like someone he knows. You can lure Revolver Ocelot into quick draw contests or make him compliment you on your gun-juggling skills. It is possible to completely miss these things on a normal playthrough of the game, but they are there for the people who look for them.
Not all easter eggs have to be as big as killing The End prematurely. One of the ones in Zone of the Enders that I liked was the fact that the colony crashes into Callisto after two years if the player destroys the colony shaft. In the game, it only takes the form of two or three lines of dialogue as part of a mission result screen. However, those few lines of dialogue are enough to acknowledge that the player has done something unusual. Adding in a lot of these kinds of details make a game feel more polished.