Why is MMR a thing?

My second account is currently gold 4, but I usually get queued with people around gold 1. Honestly, it is pretty cool, since I do learn more from playing slightly out of my league, so not complaining there yet. But is it really necessary? If I am Gold 4, why don't I get paired with Gold 4? Then, if I win, I am rewarded by going to Gold 3 for beating Gold 4. Now, if I would lose to Gold 1, I would be punished and go back to Gold 5. That does sound kinda strange to me. So, I'm not complaining, I have really good games at the moment without much salt, so that's good. But why is MMR actually a thing? Why isn't your rank just your actual rank?
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