The Trading Companies are the de facto Factions encountered in Sea of Thieves although they are not the only ones. They are, however, the only factions who sell voyages and cosmetic items.
Any pirate worth their treasure hoard knows that to survive in the Sea of Thieves, they need the backing of a powerful group. Those groups are the Trading Companies, a group dedicated each to their particular mode of "trade" on the high seas.
Every outpost from Ancient Spire to Galleon's Grave has space for a representative from the Companies, allowing pirates of all backgrounds to interact and pledge their allegiance
As pirates performed appropriate tasks, i.e. Voyages, for a Company, they gain Reputation with that Company, paving the way for shinier rewards and more risky & more rewarding Voyages.