Play is a critical part of development for young animals. Rambunctious behaviour helps them discover the limits of their bodies, form social bonds, and explore the environment.
It also makes for adorable videos. Case in point: this young elk splashing around in a puddle. Much like baby elephants at play, there is something very heartwarming about watching what will one day become a lumbering creature frolic before its body becomes too heavy for this kind of frivolous energy output.