To come together, to adventure together when their lives will be on the line, to trust each other. To, at that, learn each other's skill sets.
A session can have a reason, of course, but players can accept a bunch of total strangers at a tavern band together to go on an adventure. I seriously doubt that the vast majority of even those who would decide to adventure while knocking down the ale would do so with total strangers -- more likely with drinking buddies -- and even those are a limited subset of personality types. Mind you, the subset probably includes the people most like to go on adventures, but it's still a limitation that may not work on all stories. Players don't trouble their heads with such questions.
Having someone assemble the group works better. Still, even if the authority figure really can insist that everyone voluntarily associate with each other whatever interesting backstory and baggage they bring, there's the question of whether he would -- and whether the characters would put up with it. There's a reason why this is traditionally a group of people condemned for crimes they may or may not have committed, with a pardon hanging on it. Even if the characters must work together, there will probably be wariness. Whether it's retreating into a corner of the inn and not talking or swilling ale and bragging about monsters slain with a variable degree of honesty.
And meeting a total stranger in the middle of the wilderness or, worse, a dungeon and inducting the character into the group at once. . . . Mind you, I do have that in the work in progress, but the situation does induce wariness, the group actually knows a thing or two about the total stranger, and the stranger finds that distrusting them, or at any rate acting in a mistrustful manner, will not improve his situation.