The Purpose of you getting seeded lower than expected.
This is my opinion and theory, so take this with a grain of salt. For those of you who are new to this and don't understand why this happens, let me explain why this exists.
Riot Socrates explains on twitter that it is generally expected, despite doing well in placements, to be seeded lower than what you have hypothesized. This has been true for all seasons and it hasn't changed. Reason? There is an MMR soft reset which basically brings your MMR down a tiny bit. After that your placements are more dynamic than people mathematically anticipate or calculate them to be.
Let's get this straight. All the posts about how many wins it takes to be at a certain mmr/ LP during placements is pure theoretical math. Why? Because the official way to check your MMR and how it measures to improve is thoroughly hidden by Riot. Since they are the ones who created the algorithm it is hard to find the derivative of how it works. So with this you can say that not every win will be equal to your previous win and same goes with losses. Win streaks and loss streaks influence a huge part on how the game interprets your MMR so just because you did well in placements, it doesn't thoroughly tell you the full story due to the order of wins and losses. For example, your first 5 games could theoretically be valued more than your last five games especially with the variable mmr of the other players you play against each match. So let's say You are S5 but you lose 3 and then win 7, you could still end up in a lower tier, but winning 7 and then losing 3 could place you same or even higher tier. And this is after the soft mmr reset. So you could see some players actually get lucky and get placed higher than you did despite the starting at the same MMR and having the same number of wins and losses. I could remember last year seeing numerous pros being placed Plat, so if they climb back up you should be able to as well.
So why does Riot implement this? It's because they want you to play. They want you to have a goal and play to it. They want you to have a drive to play the game which in theory makes sense. But then again there could be arguments against it. On Riot's side, they can theorize that a certain percentage of player will play less if they met their previous goal already. Which basically coincides with with what rewards you want and how willing you are to get those rewards. Granted, I personally wouldn't want to be in a lesser tier than I was last season, so they implement this so 1) they can have more traffic and better Matchmaking for those who do have a competitive drive vs those who only want to reap rewards 2) they want you to look forward to playing the game so the new released content, while you are continually playing, gets to you (basically $$$) and this is not a bad way to keep the player interested in League and keep playing league. After all it is a competitive game. To reiterate, Riot just doesn't want you to lose interest in the game after you met your goal. Which is mostly the case because a huge population the community is in SIlver/Gold. Most people stop playing once they get there. I know I have.
So why does this proposed assumed theory not make sense to you at all. Because more than likely you will say that you would still be playing the game. At least that's what you tell yourself especially with friends in Norms. But you got to remember League is competing with many other games especially new experiences that have yet to be released, so Riot could one day lose a good portion of their audience. Why do you think Hearthstone has a new season every month? The competitive aspect of league is probably the best way and is what still keeps many players like you and me still in love with game. Ever heard of LC$? This is just another medium to influence us to get better, and keep playing the game that is rather enjoyable to watch and play. The design and intentions behind this does seem frustrating, but it's what Riot has been doing for years since release. So don't blame Riot on how they philosophically believe will keep the game alive because it makes sense to some extent.
