I like the definition of play. Not the one referring to the respect of the rules, to ancient competitions, not the game that leads to gains and losses (financial or other)... No, the first definition, the one ‘whose essential purpose is the (physical or mental) pleasure it provides’. That is, in my opinion, the essential nature of play. Play is certainly intended to be fun, entertaining, recreational, free and without much difficulty. But it is never without consequences.
Play at the Musée Gallo-Romain, Lyon, France.
Let us not switch to the futile side or the trivial. Let us look at the most personal development that can be: the one that transforms us, from childhood, towards the adult who shapes us. Through play, we learn at an early age to develop our perceptual and motor skills, to test our behavioral repertoire in the (social) context, to experience errors that do not have any consequences on our lives.
This is the role of play, which is essential and widely shared by the scientific community in neuroscience. Play is even of vital importance for the healthy development of children (Nijhof, Vinkers et al. 2018), and systematic research on play would help young patients with chronic diseases to better cope with the negative consequences of their disease and promote their well-being.
Figure from (Nijhof, Vinkers et al. 2018): Human play changes in development.
Scheme of developmental stages and (forms of) play by age. This figure gives an overview of the situation: here the categories of play are not mutually exclusive and overlap according to the child's development, interests and mood. Children can actually alternate between types of games and levels of complexity or can be engaged in different categories of games simultaneously (all references explaining this scheme can be found in the cited reference).
Reference
Nijhof, S. L., et al. (2018). "Healthy play, better coping: The importance of play for the development of children in health and disease." Neurosci Biobehav Rev 95: 421-429.