Laundry [#3]
Riot wants to boost GC teams + they are organizations which have proved they are willing to invest into the esport
This second point especially. The best possible scenario for Riot is if a group of reliable, trustworthy organizations are willing to invest into the esport. That's what's most beneficial to Riot from both an engagement standpoint (orgs have followings, and those followings mean more attention is brought into tier 2) as well as a monetary/longevity standpoint (like I said, orgs will continue to invest into the game). This is the ideal scenario for Riot, and it's the exact same reasoning as to why franchising is a thing.