It celebrates a basic principle of Jewish teaching – the belief in miracles.
True, some tend to regard miracles metaphorically, seeing them not so much as supernatural events but the surge of a miraculous human spirit of valour.
This is all very well, and it is quite likely that great events like Chanukah would never have happened without Maccabean heroism. Nonetheless, what the miraculous really means is the incursion of God into human history.
When all logic would argue that a cause is impossible and that the few who stand for justice will never prevail against the many, the miracle demonstrates that God is still in charge, and when the time is ready He acts.
The belief in miracles tells us that we must never despair. In the end God does not allow his world to be hefker – ownerless and out of control.
This, if we read the Chanukah story correctly, is why the sages in the Talmud play down the military victory, which represents the merely metaphorical concept of the miraculous, and emphasise (Shab. 21b) the little jar of oil, which is not a human deed but God Himself acting in history.
This is also why the major mitzvah of Chanukah is the kindling of the lights, expressing our faith and hope that the Almighty will bring the light of redemption to an often dark world.