I wrote this in response to another thread, but it got long enough I thought it deserved its own post. Snippet of original comment, something I read on here quite often:

Have people actually place in their real rank and not lower than real rank. This just makes the game super bad for everyone. People who win all 5 placement matches arent Silver or low Gold. People should climb and fall ranks much faster than right now, especially as every rank essentially has 3 sub/ranks anyways.

This is exactly what MMR does, climb and fall faster than rank, but removing it(or making it match rank exactly) will create more problems than it solves.

Because of a statistics thing called variance MMR often ends up much higher or much lower than a true representation of the player's skill level.

Therefore, in order to accurately determine a player's rank, multiple games(far more than 5, more like 50+) must be played to narrow the confidence interval. This is the explicit reason why each rank requires 100 RR.

Let's say a player belongs in Gold 1, and the placement matches were neither lucky nor unlucky for you and Riot estimates your MMR at Gold 1. The game will assign your rank, for example, as Bronze 3, halfway to Silver 1. However, we cannot be sure about this, because we only have five matches worth of data.

Now what happens is you play another 50 matches, against people who have MMR close to yours, Gold 1. If you hold your own, not top fragging every game, not bottom fragging every game, then on the 50% of matches you win, you'll gain on average 30RR while on the 50% losses you'll lose 15RR. (Remember these are example numbers used to illustrate) This equates to a RR gain of 7.5 per match. (-15 * .5) + (30 * .5) = 7.5. After 50 such matches, you'll have gained 375 RR. Meaning you will go up from Bronze 3, through all of Silver, and be +25 RR into Gold 1.

Basically this means the system is doing the opposite of what you're suggesting. It isn't putting people who belong in Gold 1 in matches with Bronze players. It's goal is to put a player who belongs in Gold 1 in 50 matches with other Gold players, except with a starting rank of Bronze/Silver. People who have already played 50+ matches and are still in Bronze/Silver, will never play against this player.

Now let's look at the case where some bad luck is involved with the 5 placement matches. Let's take a player who actually has the skills to be in Plat 1, but because of sheer bad luck(or maybe they were drunk?) only performs well enough to beat people who are Silver 1. Now in this case, their MMR will be that of someone Silver 1, and the game will assign them a rank of Iron 3(!!)

When they begin to play their next 50 matches, the game will match them with teammates and opponents who are in fact Silver 1 even though they themselves have an Iron 3 rank. But this player has the skill to be in Plat 1, they were unlucky in the initial placement matches remember!? They're going to totally stomp the Silver 1 players and top frag almost every match! The silver players will take to reddit to complain about the smurfing problem, when the truth is this "smurf" just had unlucky placement matches and the game doesn't have enough data to accurately match them to plat 1 opponents yet.

Since however, they are winning almost every match and probably MVP 80% of the time, they'll gain 40+ RR each game. Their rank will rise extremely fast and their MMR will be rising as well so that after just another 10-15 games, they'll be playing against Plat 1 players(MMR) and will have already ranked up out of Iron, through Silver and into Gold(+RR). Now after completing another 40 or so games, they will end up in Plat 1 where they're supposed to be. Only 10 or so matches of "smurfing" needed to occur for matchmaking to correct the original Silver 1 MMR placement to Plat 1.

Now there's obvious downsides to that, those 10 matches probably sucked for the silver players. But there's not much you can do about it, that's how randomness works. It takes more than five matches to be confident in a player's skill level. But having an Iron 3 ranked plat player play with Silvers for couple of games is better than the alternative of tying rank directly to MMR and here is why:

Consider the opposite scenario where someone gets very lucky on their first 5 placement matches and performs much better than their true skill level. If they're a Bronze 3 player, but got lucky in their games and get assigned an MMR for Gold 1, then they'll be matched against Gold 1 players but with a rank of Bronze 3.

So in this case, the MMR is high, but the rank is correct. They're going to lose 80% of the next 10-12 games the play and their MMR will drop like a rock to Bronze 3 because they don't belong in Gold 1 even though their starting MMR is Gold 1. But their visible rank won't drop. Or if it does, only to Bronze 2 because the higher your MMR relative to your rank, the less RR you lose on a loss. If Riot directly correlated MMR with rank and gave this person Gold 1 to start with, imagine how they'd feel going from Gold 1 to Bronze 3 in the span of 10 matches? They'll uninstall the game and write hate mail to Riot, cursing the developer's ancestors as they do it. That is why MMR exists and why it is a good idea.

And to be fair, Riot could do a hell of a lot better job explaining this than they do.

And now the footnote, yes, this can be abused. If you make a new account and you're very good, but deliberately play like crap in your placement matches, then it will give you an MMR of Iron 2 and a rank of Iron 1. And then you can play with noobs and get 50+ kills for about 20 matches until the game fixes your MMR. But the fixes I most often see suggested on this subreddit, while potentially and only partially alleviating this abuse problem, would also create far worse problems for legitimate players.