Most folks who like MMR do so because we have experience with MM systems which do not attempt to account for player skill. If you want to hear about some of the pitfalls of Experience, "stats", Or 0 MM systems let me know.
Trying to avoid getting side tracked so lets get to your main question.
What does this accomplish? Why is this a good system?
Basicaly
The benefits of MMR is that eventually it gets to a point it creates a workable range to use as a representation of skill. A con is that its not instantly correct, and that quality only increases mostly as you travel UP the ladder as playing to lose is a fairly easy thing to do with out requiring much effort.
What your MMR value represents is a range built as your mean average of game results + who you played with/vs to generate those results. When its accurate its very helpful to sort other players in addition to making a smaller gap between team skill. When its inaccurate the level of inaccuracy generates a negative match.
Uncertainty is represented with by a K factor, it lowers in relation to # of games and/or Ladder position. So, your 
for example was in Provisional games. That's to say placement matches. Now these MMR systems typically have a few rating protection features in place, but those protections have to do with the post game adjustments to protect your rating. They don't impact the quality of the match which tends to be what players care about. So some times in a really shitty match, if you have a teammate with a low # of games played that single handedly cost you guys the match. Your just part of the calibration process for that account. Just like the games you are VS a terrible enemy that gave your team a free win. And the odds are a good deal higher you get more wins from baddies then you do losses. If your upset by the inverse... your acknowledging there are players above you in skill that make more impact then you can... so at that point your gonna hear folks say some variation of "working as intended"
Riots implementation of duoing makes for issues, Your Silver 1 and other Bronze player for example have a wide skill gap so one way to balance that is to make the enemy a more uniform average, player population will impact how MM can try to account for Duo ranges. I actively dislike the current duo system and feel it needs to be off MMR not Tiers. I can rant about that a lot so.. unless you need more info lets move on.
End of the day what happens with MMR is it does sort folks out by skill over time, it never locks some one into a range so getting better (or worse, Champion nerfs and reworks are a thing after all.) Will start showing up in your rating no matter how many games you have played But at the same time if you never improve then regardless of how many games you invest you wont climb. A positive thing if your trying to rate skill.
When its accurate your MMR serves to represent a probability theory concept built around the distribution pattern. Gonna use Elo for an example here. 1200 vs 1400 would see the 1400 player expected to win around 3 to 1. Then if its wrong it moves both players in different directions. Probabilities nifty in its ability to help calculate future events. If I asked you to roll 2 dice and there was a bet for if you guessed what number would be rolled? You can guess any combination of 2-12, you can see any result out of a single roll. But throw them a million times? 7 shows up far more often then 2 or 12. There's a reason "house always wins."
Many of the numbers used are arbitrary and only have meaning in the context of the system. Pretty normal for raiting things. a 10 point performance can be good or bad depending on context. 10 out of 10 is best. 10 out of 100 is bad. There is no inherent meaning for 8 to an Earthquake separate from the system . Richter scale could just as easily function by counting up to 100. Same way 1 Pound can be represented by 16 ounces or the words Dog and Perro both refer to a fuzzy household pet.
So recap.
Its more accurate for sorting skill then many other systems available.
Its functional, eventually.
The frustrations of individual matches don't ultimately prevent its functionality.
Its objective.
Its testable.
It does not magically make a game more fun... Its not remotely concerned with the game itself.
Subjective systems are fairly hard to automate properly with out some Elo system as the biases.
Elo and MMR are awesome, but this does not inherently mean that Riots application of it is awesome. Not every bell and whistle is a positive, Duoing for example is terribly implemented if you care about your majority MMRs individual match quality.
