When playing video games there is always an objective to complete after completing the objective the gamer is normally rewarded with in game currencies or items. This gives the player a goal to achieve and once they have completed it they have a sense of achievement especially when given a reward.
Many games use this feature with person vs person games allowing players to see what other gamers have obtained while playing the game. If witnessing someone with a better reward than yourself you may create a new goal to achieve and obtain that reward.
Some games give the player an option to purchase items and in game currency with real money. This is sometimes acts as a quick access to that reward as the player would of had to play many hours to achieve it.