A lot of folks will tell you its to slow the ranked climb. Truthfully it accomplishes a few extra things. It helps the system match folks to tiers their MMR is close to. I feel a good range of being close to skill where it becomes more difficult to climb is 2-4 divisions depending on what mmr. The Promos hurt bad luck but good players, but they are good so it assumes they will make it back to promo and win 1 out of 2 games after it gives them the free win. This step is added to help reduce bad luck, having promos can slow a lucky players climb. People getting placed to high are unfun, they cant compeate and they drag their teammates down with them.
Not only this but the promos allow for Division skipping to get players the system had rated to low into a closer mmr.
I encourage you to look at systems for why they exist. I hate DLC, passionately because its just abused so much, and a big reason we have it? used games. Devs had to find some way to keep a profit when places like game stop would make a profit off a copy of a game multiple times and devs got none of it. I get why people did it, I still don't enjoy it, but I get it.