Project Booster Fun was pitched to the public as an initiative to make opening booster packs more exciting. But to the discerning eye it was an attempt to replicate the “loot boxes of cosmetic upgrades” model from video games in MtG. Prior to Project Booster Fun there was one type of booster pack, used for the limited formats, and four rarities (three prior to the introduction of mythic rares of course), plus parallel foils. Project Booster Fun introduced two new types of booster: set boosters, and collector’s boosters. Neither of these can be used for playing limited, their allocations of cards and rarities are different than that of draft boosters, and they cost more than draft boosters. They also introduced several new types of “premium treatments” which are like foils in that they are functionally identical to the regular treatments, but have alternate artwork, frame designs, and foiling. I frankly don’t remember how many types of these there are, but they are found at greater frequency in set boosters and collector’s boosters, and some of them can only be found in collector’s boosters.
Mythic rarity may have foreshadowed that they were looking into messing around with rarity as a way to increase profits, but Project Booster Fun was where they really went all-in with the idea. Here’s a link to the announcement if you want to read up on the specifics yourself:
Project Booster Fun | MAGIC: THE GATHERING
EDIT: Huh, I guess I misremembered; set boosters actually came later. Still, they were a further continuation of what Booster Fun started.