The party is an almost inevitable feature of any CRPG. The genre, whether computer or pen & paper, is predicated upon a group of heroes (or whatever) setting forth together to battle the forces of evil (or whatever). How partying works within an RPG, and to what end, then become fundamental to how the game itself plays.
There is no one true approach to the party. There are some extremes, with enforced partying at one end, and no partying at all at the other end. Online and offline RPGs take different approaches. In most offline games, the party is required, though only one character (yours) actually matters. The party is a necessary part of playing the game, but your role as the player is two-fold: develop your character, and manage the party, much like a sports manager or coach. Get the balance of skills and abilities right, and you