what else would be the point?

Well, what was the point of every Mario game prior? World wasn't about collecting dragon coins, to my memory. Platformers to that point had been effectively obstacle courses about overcoming the challenge laid out for you. They didn't need a skinner-box rat-and-cheese number-go-up extrinsic motivator to get you to play, just traversing the levels was fun because jumping on and over stuff was fun, defeating enemies was fun. Top examples are Mega Man X, the first Castlevania, indeed Super Mario Bros 3, Sonic the Hedgehog 2, Kirby's Adventure. None of these games needed to be "GRAB ALL THE STUFF", anything you could collect in these games was either supportive of the core gameplay (subweapons and hearts in Castlevania, armour and sub-tanks in Mega Man X) or purely for score. Gating your game's progression based on arbitrary collectibles was an absurd idea built purely to pad a game, and you can see what a bad idea it was in Symphony of the Night as well. It's a bad bit and I think there's a good reason most games dropped it. The level design in SM64 and its ilk also get warped to this purpose and mean that without their silly collectables there would be literally no point. A Mario 64 that's just "get to the end" would be an hour long game. And why*? Because they decided that platforming being fun on its own wasn't enough somehow.

*it's cartridge space, again. they padded the runtime because more levels wouldn't have fit lol