Moisturising protects the skin from the exterior environment - wind, sun, cold, or dry conditions will age the skin prematurely if it is not protected in some way.
For dry skin, rich moisturisers are required to keep it supple and elastic. For oilier skin, only a light moisturiser is needed, otherwise the pores can become clogged and heavy. Some moisturisers have been produced to tailor to all skin types - you may find the need to apply more or less according to your skin's needs.
There is some debate over whether you need a separate day time and night time moisturiser - Pukka's advice is that it comes down to your personal preference, but for more information, visit Beauty Tips.