Confused about the logic concerning placements

Season 5 was my first season, so pre season 6 was my first pre-season. Back in pre season, you were still able to ranked and I was curious as to what the point of it was. I was told that it was a good way to climb and to get a better MMR or whatever for when the season comes. That sounded pretty cool, I can now rank without rank anxiety and possibly be in a better spot to do real rank than bronze 4.

But I see now that the placements destroy your rank and puts you really low, so looking back now, what was the point in even doing preseason rank at all? Was that all for nothing?

Just curious, I am an ignorant pleb