Help me understand placements please

Why do we even have placements each season?

I finish bronze 3 last season and went 6/4 in placements and got placed in bronze 5.

That doesn't make sense.

If i went 1/9 in placements, id be like, yea maybe i deserve that.

Note: to the mod who deleted my original post, i initially was going to place in game play but thought that it was actually game play related like spells and stuff