What binds the party together?

Over time, I've realized that there is a great advantage for everybody implicated on a campaign when the PCs party belong to an established group in-game, with an implied goals and sometimes, even a roster for backup PCs.On our cyberpunk campaigns, it was the gang. Everybody starting as gang members gave you a drive and an identity instantaneously, leaving your character background story for whenever you wanted (or never). You never had to think about why your character was adventuring with the rest of the party (even though you could if you wanted). Blades in the Dark is built on that, too. On the game I wrote some days ago, PCs are assumed to form a group of Vampire Hunters.They are not as evident, but Shadowrun and OSR D&D work nice for that, too: Your character starts as a gold seeker first, background second. It is very easy to work a game with no preemptive thinking as long as you start the game in the door of the dungeon (One of the best advice I've ever heard). By the time the…