Can someone explain to me the reasoning behind the Riot ranked system? (Or link where to find info)

Mainly I want to see:

• Why they have promo's (which are 300% more toxic than regular matches)
• Why they remove the "free win if you lose a promo" after a certain rank

Overall I like their ranked system. If there's something I don't like I can usually get behind it if there's some good reasoning, which I'm sure they've got, I just can't find it :D.