That’s how the sleazeballs who run these rackets can technically claim they’re not pyramid schemes: they’re not having their own suckers recruit marks on the next level down in a direct cash exchange, which would be illegal. Instead, they mask things by having their suckers convince the marks to buy “starter kits” of whatever overpriced garbage the grifters are selling and then giving the suckers “sales commissions”.
ETA: there may not be an official requirement to buy the junk if one wants to join an MLM, but without buying the junk to supposedly sell or be a distributor of there’s no real point in joining in the first place.
With a pitch like that, you should really buy a Beginner’s Pak of Gracchus’ Tiger Repellent. You’ll make a fortune, especially if you identify friends who could also sell it.