perhaps the purpose was didactic: when a child gets interested or wants to help those tribe members making stone tools, it is not yet aware of difficulty level

perhaps the purpose was didactic: when a child gets interested or wants to help those tribe members making stone tools, it is not yet aware of difficulty levels of different materials. teaching a newcomer to make a proper tool from a more annoying material, may make their skills more robust when relaxing to easier materials later on. think of all the skills you had to learn in your own education, and how little of it you actually use today, or at least how little you think you use of it today.