It is a hallmark of contemporary dance owing to the traditions established by its pioneers like Isadora Duncan (although I'm not sure how many young performers today realize this)...
Duncan and others wanted to get the art away from what they considered the rigid formality of traditional dance, notably ballet which she found to be mechanical and stiff. They took their inspiration from natural movements such as the ocean's waves, trees swaying in the wind and animals running gracefully.
Duncan also took inspiration from classical Greek statues and artworks, eschewing highly formal dance costumes in favor of loose wraps, toga-style dresses and dancing barefoot.
All of the above were designed, again, to move the focus of dance away from precise lines and movement (which artists like Duncan found to be mechanical and artificial) to a more naturalistic and flowing expression.