Placements - Do they matter (after first)?

I'm curious as to whether they actually mean anything after your very first season of placements, as the system seems to just throw you wherever it wants in the following season, based off previous season MMR? (my first season was last, I was placed Silver 5 in that, and my placements were something like 4/6, from memory, even more confusing because as follows;)

I went 8/2 in my placements this season. The 2 losses were due to 1 feeder in each (like proper feeders, 10 deaths in 5 minutes type of deal). The 8 wins were almost entirely complete domination, 5 or so surrenders, I carried about 4 of them. I checked, and the people I beat in these games were Bronze 2 - Silver 4. I was placed in Bronze 3.

It just seems odd as, if my MMR was diminished with season start, from previous season (was silver 4 I believe), then I would be matched against similar MMR players. I would further assume if i'm going up against Bronze 2 - Silver 4 players who are similar MMR for placements because of this, that beating them would land me at least equal to them in rank/MMR, if not above them. But I got placed below all the players I beat in placements?

I don't really care about being placed in B3, with enough grind i'll get out. I'm just curious if there is actually any use to placements? I would assume that if they don't actually meant anything after your first season, then people should just be placed at whatever position the system deems you to be in, without the need for placements, in the following season?