Could someone calmly explain why MMR is a good system to me?

We got a low winrate Bronze 3 in a game where no one on the other team was below Silver 3. Yes, they got completely dumpstered. We also had a 48% wr Bronze 1 (again, no one on their team was below Silver 3), and yes, they got completely dumpstered. I know a lot of people love the MMR system, so could you please just calmly explain why to me? It routinely creates terrible mismatches where the outcomes are certain, and ones that anyone just looking at the rankings could predict with 97% accuracy. What does this accomplish? Why is this a good system?
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