Typically ranked soft-resets drop your MMR slightly (we aren't sure by how much), and increase your variance to some degree (we aren't sure by how much). You shouldn't be seeing the same quantity of ranked points gained or loss, unless perhaps you're losing to a worser team, winning against worser team, or you didn't play enough games last season for your variance to shift at all (unless, I'm woefully mistaken).

So after placements, you're essentially guaranteed to be slightly lower, exactly the same, or slightly higher than your previous rank. However, your variance will remain higher for some time (which is good, because it's easier to rank up).

It's hard to speak of it in terms of a a distinct "purpose". It basically has you reestablish your current MMR, or even jump a little higher with ease. Then allows you to climb higher faster until you're variance drops back down; which makes sense, since you've probably improved over the previous season, and shouldn't have to play 50 games to move up 1 division. The downside being you'll also drop faster if you perform poorly.

It's possible that Blizzard stopped refreshing variance when they tied MMR to rank, but I can't find anything that would lead me to believe that. Not refreshing variance would be a horrible idea, because players with, let's say 1000 ranked matches, would have to spend a ridiculous amount of time to rank up even the tiniest bit.