What purpose do promotion matches serve, on a practical sense?

I seriously would love to hear the community's opinion on this.

I'm a scrub and I'm pretty comfortable at being in the low rank that I'm at. I recently lost my promos but it's fine. This is not me saying promos are keeping from my "well-deserved high rank" or whatever. My games have been pretty competitive so far and I lost my promos because I screwed up.

With that said, it got me thinking: If one of my team mates were on their promos as well, I basically just robbed them of their rank-up and that can't possibly feel good.

You'd think the designers of LoL want people to tilt in Ranked. I'm willing to bet there's no bigger source of conflict between players ingame than promotion matches, where players are naturally more tense and are far more likely to tilt at their team mates.

I'm a seasoned relatively high ranked player in other games (mainly fighting games) and I sincerely don't understand. Ranking up is already a stressful and time-consuming experience as is, why add an additional system which is only going to make a portion of players feel bad, while the winning team gains nothing in return? Why does this system exist in a game that's so prone to team mistakes and snowballing?