Why is it important to include divergent views (counterclalms) when you write a persuasive essay?

Offering a counterclaim and giving enough evidence to disprove that counterclaim strengthens the argument by reassuring the reader/audience that the you are well-informed and able to discern multiple perspectives. It's like when you're trying to convince your parents to let you go out with your friends at night or on a school night, you should state why you want to go and you should also acknowledge what they're trying to say otherwise they'll think that you're not aware of the dangers out there.