Social perception skills generally involve two things. First, there is the skill of forming an opinion about another person. Second, there is the skill of explaining and understanding our own behavior and that of other people. Both of these aspects of social perception are involved in being an entertainment agent.
An agent must form accurate opinions about other people. Agents have to deal with all sorts of people, including their clients, people who employ their clients, and people who want things (like interviews or business deals) with their clients. If they are going to serve their clients well, agents must have accurate opinions about all of these people. They need to have a good sense for how to deal with all the people they come in contact with.
Once they are in contact with these people, they need to understand the other person's motivations and behaviors. For example, when negotiating a contract, they have to be able to understand why the other party is asking for a certain aspect of the contract. They will also need to understand when they can push for more and when the other person has reached their limit.
In these ways, entertainment agents need good social perception skills.