This is the continuation of my last theory, previously I stated that the Ender Dragon is the Endermen's creation, but the reason why it was created hasn't been told yet. So now I am gladly to tell you that I have discovered the reason :D Let's get to it.

As you defeated the Ender Dragon, a portal with the Dragon Egg on top and the portal leads to the End City will open. What if I say that the Dragon was created to defend the gateway that goes to the End city and the portal with the Egg on top is nothing but a distraction?

I know I know, Dragon Egg is the rarest item in Minecraft, it's nonsence to say it's a distraction...But is it? Getting it is hard, I admit; however, does it gonna be any use? Unlike Nether Stars, they couldn't be used to make Beacons, which give buffs to yourself and others... So, it is not worth it to get, unless they were created so the players won't put their attention in the portal which floats in the sky.

Why do Endermen need to trick us? Likely, they want to hide something. If you already go to the End Land, you will see tons of structures, tall plants, and many overpowered loots. Let's look at our world, there're some places that is prohibited to come to because the government wants to protect the beauty of that place or something; so the same as Endermen, they want to protect their place. My thought is, in the beginning, you could go to the End Land directly through the Portal, but then when players got greedy, they took resources from the land, killed any Endermen who got in their way. After that, the Endermen got angry, created the place where you must go through first and called in the Ender Dragon to protect the portal. In case the dragon got defeated, which it most likely will be XD The Endermen created a distraction, you know what it is already, which gives them a useless item (will be useful for exhibition) to make them think that their job in the End is finished and send them back to the Overworld. Although they want to protect their land, I think they want to protect one more thing, the Elytra, this maybe the reason why players fight them for this item (Does anyone don't want to fly without creative mode?)

Additionally, this also explains why Endermen kill us when we look at their eyes because they'll remember the past, when we came and killed them. Leads them to scream to our face and rekt us :).

In conclusion, Ender Dragon is summoned to make the players think that it is the final boss, instead of thinking it is a distraction. So there you go, the reason why Ender Dragon was created. Hope you enjoy this theory! I also want to say hi to the one who makes thoeries, too :D Keep up the work guys!