As in real life, most games are build with an econimics system. Some games have more complex systems than others, and some tend not to lean too much on it.
One game, for an example, is World of Warcraft. In this game you gain money buy completing quests, or buy killing humanoid opponents. You can also find items on enemies you then either sell to an AI vendor, or you resell it to another player in the game. It's quite simple, and it's easily done. Another example would be eve online. This game, however, has a lot more complex economics system. In this game, you can actually take the part of a trader, not doing much more in the game than buying and selling things to other players.
In these new worlds, money plays a great role, as they do in real life. You need money for food, clothes, armor and weapons. Even mounts or space ships can be needed, and as such, a much needed investment.
Though, admittetly, it can be quite tiresome to keep grinding items, or monsters to gain enough money to make your computer life worth living, some people have acutally made quite the impressive amounts. One guy acutally made more than a million ingame gold in world of warcraft.. and for those who don't know how much that is... I'll tell you : It's a lot!!!
You can read more about that guy by following the link below.
But all in all, I think economics are needed in these games, because we use them as a second life really. A more relaxed life of course, but we tend to accept things closer to our real life. Who would want to play a game where you would get everything for free?