Social interaction is more important in the Worlds of Adventure campaign than in many other D&D campaigns. "Social interaction" doesn't just mean "talking to NPCs" — that's only a small part of it. It covers many different aspects of interacting with the inhabitants of the worlds your characters visit, and the people and creatures you meet in your adventures.
Morale: Monsters and other opponents rarely fight to the death; most creatures aren't fanatics, and would rather flee than be killed. If the fight turns against them, or if they think you're too strong to fight, your opponents will turn and run. They may also surrender or try to parley, if they think you'll spare their lives. The morale check rules determine if, and when, this happens.
Reaction Rolls: "First impressions are everything," the saying goes. The way that your characters' interactions go, with people you'll encounter, depend on those people's impressions of your characters; that, in turn, depends on what they've heard of you, your behavior, what you look like, your race, your appearance and that of your companions, and so forth. People's reactions affect how they'll treat you, how they'll act toward you, and every other aspect of your interactions with them.
Reputation: If your adventures are successful, words of your characters' deeds will likely spread far and wide on the worlds you visit (and even beyond). The nature of that reputation will depend on your actions, and will, in turn, affect how people see you and how they treat you. Your characters' reputations affect all other social interactions — every use of a social skill (like Diplomacy?, Intimidate?, or Gather Information), reaction and morale rolls, your Leadership score (see below), what sorts of people gravitate toward you, and what sorts of opportunities for quests and adventures you find.
Leadership: When your characters reach a certain level of renown, people will start wanting to associate with them, to learn from them, to follow them. (Unlike in Pathfinder, this doesn't require taking a particular feat.) If you have some permanent base of operations, you can found an organization — a thieves' guild, a temple, or something else, depending on your class, your reputation, and other things. The people in your organization can do a variety of things, including craft items, do research (alchemical, magical, lore, etc.), investigate rumors, provide information and knowledge, go on missions, spy on your enemies, build things, and generate opportunities for quests and adventures.
Alignment: Your actions affect your alignment, and your alignment affects how people see you (see Reputation, above), which in turn affects how they treat you (see Reaction Rolls, above).