The existence of Ewoks in Return of the Jedi seems to be a fairly controversial point in the original trilogy. One of the main issues I see arising from them is the ease in which they defeat the Imperial squadrons defending the shield generator, and how, stemming from that, they are responsible for the Rebellion's victory at Endor.
I've seen many of these opinions expressing disappointment that a technologically advanced military troop could be defeated by a tribe of "fluffy teddy bears with sticks and rocks." While I can understand the frustration that people have with this, I like to view it a bit more symbolically and don't suffer that same disappointment.
I view the victory of the Ewoks against the Empire as justified merely because of the differences it serves to illustrate between the Rebel Alliance and the Empire. The Ewoks encountered both Imperials and Rebels before and during the events of Return of the Jedi.
In typical Imperial fashion, the Empire destroyed an Ewok village to build the shield generator for the Death Star II. They then continued to subjugate the Ewoks, preventing them from returning to the areas they lived in that were now in use by the Empire and confiscating their hunting equipment, deeming them dangerous weapons.
In contrast, when the members of the Rebellion were confronted by the Ewoks, Luke urged a peaceful course of action. He wanted to treat them like people who simply didn't understand the situation, not like savage animals. With the help of Threepio, he was able to come to a peaceful solution and befriend the Ewok tribe.
So while the Empire offered an unprovoked assault on the Ewoks, the Rebels were ambushed by them and still chose to look for a peaceful solution. It's this situational difference between the Empire and the Rebellion that makes the Ewoks' assistance to the Rebels mean something to me. It becomes emblematic to me of the peace and understanding the Rebellion is trying to promote in the galaxy, in direct opposition of the Empire’s tyranny and oppression.
Ultimately, I see it as a symbol of the reason the Rebellion deserved to win the war; they treated other beings as equals and strove to bring freedom and peace to the galaxy. It emphasizes an overarching theme in Star Wars, that no matter how limitless, or invincible, or unrelenting an evil is, it can still be defeated by the powers of friendship and love. There has never been a darkness so dark that it could not be driven back by the light.
