I think for games with levels, there will be easier intro levels and it can take a little while to ramp up into something representative of the game itself. Often longer than any game anyone would be able to make in the time limit of a game jam. So you wanna make levels the show off what the game can do/be, but there needs to be at least a somethong to ease players in and let them get the hang of the mechanics. Can be a tricky balancing act.