[Legolas]

I think a key point is that men (or elves, dwarves, etc.) cannot defeat a force like Sauron's (especially with Sauron, an Ainu, in leadership). Gandalf the White and the Eagles are intentionally placed; they are reminders that man cannot do it alone, and that Eru has not simply created the world and ignored it since. He does not leave it without his own representatives and influence, when necessary. This also happened at the end of The Silmarillion - Eonwe and his army have to give the Children a push and accomplish things that would've been unsuccessful otherwise.

In both instances, the Children were matched with adversaries far greater in strength. I think that fact is unsettling to readers.

'Over the bridge!' cried Gandalf, recalling his strength. 'Fly! This is a foe beyond any of you. I must hold the narrow way. Fly!'

'What hope have we without you?' - Aragorn