The more a user or player invests in their part in a game, the more they are likely to keep playing, come back to and arguably, enjoy the game as a whole.
A MMO/Second Life requires a high user investment, this is a barrier to entry for many people, although once you have started the investment in your avatar, character or world it is far more compelling an experience.
From a design paper on creating virtual pets …. the trick is to create a positive feedback loop on the user investment in taking care of the pet. The more the user has spent time interacting with the pet the more it is crucial for him that the pet does not die or run away and matures properly. The initial investment may simply rely on the money spent to buy the pet. Then, the relationship merges from this self-reinforcing dynamic.
The also argue to include uselessness in to the design of pets, to encourage a dependence on the player.
Rewarding user investment, psychologically, the more someone is invests in a game the more valuable the benefits and rewards that come from it are.
Clan games & RPG’s expand on user investment in characters to include guilds, clans & any number of social groupings.