Why even have promotional games?? some input to relax my raging mind

Okay, I am probably going to sound like every other joe that gets on here and brags about stats and skill while only being in silver but this is pretty much a vent/rant about the uselessness of promotional games for players over 2-3 seasons into league. I work hard as hell to get from bronze 5 to silver 3. It took me all season and most the preseason. I get that accomplished. Go 8-2 in ranked (stats don't matter, but I dominated every game but the two we lost. I bring this up for something I will say later) get out and get put into silver 4. Come on Riot. Whats the point. you know what 8-2 gets me in a regular season. Moved up to the next tier or division. What is the point of playing to get better when you get nothing but moved down and wasted time. If i didnt have to play promos id be in silver 2 on my way to 1. I have worked my ass off at getting better and have done that, kda has improved dramatically, teamwork has improved a ton. overall my game has got better. It showed in all my stats and games. I get put back lower then what I was, with more wins and better play as a whole (not just personal stats). I don't get the point. Maybe someone on here can explain it to me so I can understand why we still do promos 4 seasons into playing the game.