The presence of representatives of all free peoples of Middle-earth suggests that this is their ultimate challenge to Sauron and the moment in which they demand him to answer for all the pain he’s caused them. He sends his ambassador instead of going himself, implying both that he could be threatened by the army, or that his plan relies on his own distance from the battle. That the Mouth of Sauron has forgotten his own name after serving Sauron shows that Sauron is more focused on amassing strength in numbers rather than valuing the individual traits of his soldiers.