Why is maintaining a circadian rhythm important?
In theory, having a circadian rhythm should help anticipating daily environmental changes (light, temperature etc.) so that the metabolic performance is maximized.

In practice, mice chronically exposed to environmental light-dark cycles with a period length dramatically shorter or longer than that of their circadian clock are prone to become obese. Moreover, animals kept under short cycles die earlier than mice kept under normal or extended cycles. So, chronic circadian disturbance by a shortened light-dark cycle increases mortality (at least in mice). In humans, shift workers are likely the population that is more susceptible to circadian cycle derangements.

Ref: Park et al., Neurobiol Aging. 2011