The reason they brought in MMR is related to the core philosophy of rating systems.
To start with, we all have a theoretical “true rating” in every competition imaginable. What rating systems does, is try to get participants to their “true rating” in various ways. Because even an absolutely new person that has never played the game before, doesn’t necessarily have a skill level of 0 rating. They might be closer to 1300. Or 2000. And so on.
So what the separated MMR does, and why Blizzard said they brought it in, is to make the better players reach their rating faster than they did in TBC when the CR was the MMR and you couldn’t win more than the opponents lost, etc.
Which it does, but it’s also inherently more volatile because of that, when compared to the Elo system which they based the TBC rating system on. The current system isn’t based on Elo, it’s much closer to the Glicko RD rating system along with Blizzard’s own tweaks here and there.
So that’s why the game has a separated MMR from the CR, and how points won/lost is relative to your MMR and its volatility.
Personally I prefer the Elo system they used back in TBC though.
Just a side note
It’s easier to wintrade in the game’s current version than it was back in TBC. TBC had a problem with feeding wins though, which is slightly different and was because of the small matchmaking pools in some of the battlegroups.
Wintrading as it works now is when two teams, usually on really high MMR, play during hours of the day when there’s the fewest amount of players queuing for arenas near their MMR. This makes it so they get matched up with each other over and over again.
So what they then do is giving each other wins in a controlled fashion, in a way so that both of the teams ends up with more rating than they started with before those games.
This is only possible because of the way the MMR works and how you can win more points than the opponents lost, and vice versa. They’re literally “creating” new rating points out of nowhere when they do that.
Which isn’t possible in TBC, when you could only win what the opponents lost and vice versa. So you could keep trading wins to each other, but one team would always end up with more while the other team would end up with less, compared to when they started doing it. Since they’d just be passing the points that already exists between each other.
In other words, they’d “feed” one team.
It’s still possible to feed in the current rating system, but it’s less efficient than wintrading. With a large matchmaking pool as we have now, it’s also much harder to feed unless both teams are already at a high MMR.