If you only gained 20 LP and lost 20 LP a game the hidden MMR would be fine to disapear

But as it works Riot instead wants players to stay near a 50% win rate. It is not hard to understand. If someone loses 80% of their games they will not feel like playing this game after a while. It is a marketing scheme to keep more players around.

Say a bronze 5 with no understanding of farming, ganks, or even what items to buy on Ashe because the tutorial told them to buy thornmail wins 45% of their games even when in the lowest 5% of players skill wises. They are more likely to want to keep playing.

As such Riot instead of granting a game of silver 2 vs silver 2 where one plays like a bronze 5 and another players like a low diamond while the rest are around a high silver skill level decided to have players in games based on how they consistently preform. The hidden score pushes players who are better than their displayed rank to be facing better players but gaining more LP for a win and lose less for a loss while players worse then their displayed rank to face worse players and gain less LP for a win and lose more LP for a loss.

It is a plan to try and make even the worst of players have a reason to keep playing.