Why do "promos" exist?

I dont understand why this concept is better. Shouldnt there be pure thresholds for when you're considered "Silver" or "bronze"?

Example

100 - Bronze 5 200 - Bronze 4 300- Bronze 3 ... .. .. etc etc. People get super tense and butt hurt about promotional games. Just makes LP thresholds instead of "leagues" where you need to enter 3-5 round promotional series that have an increased meaning.