Here's my crazy theory. The sea of clouds is a place where a single misstep could send you plummeting off the edge in to the clouds themselves. This could be especially dangerous at night, where visibility is much reduced and there all sorts of other creatures lurking about that could threaten the native Vanu. So the dance with it's precise motions and appearance of psyching oneself up, was done as the sun dropped below the horizon to signal the approach of nightfall. Thus the dance became a way for the Vanu to practice precise movement and psych themselves up before going out at night. Over the years however it became a dance that symbolized that courage and strength of the Vanu and began to suffuse other aspects of their lives until it became a central part of their culture.