The writer and literary theorist Kenneth Burke came up with the idea of "identification" as a form of persuasion. Basically, to be persuaded, you need to identify with the speaker. (Source)

This is basically the reason you want to be best friends with Scout Finch, want to give Holden Caulfield a hug, and feel deeply uneasy when reading The Stranger. You identify with these characters (or, well, the author's use of the first person) and you feel persuaded.

Wilson's use of "we" at the end of his speech is an innovative instance of identifying with a global cause. His goal is to get the rest of the Allies to think of themselves as the "we" that includes the United States and other free nations.

At the time, this was a pretty radical new idea, and foreign leaders recognized its innovation. Wilson's speech laid the groundwork for the creation of an international community. (Source)

Yep: without ol' WW, we might not have been singing "We Are The World" in 1985.