The grass is still green underneath and can use all the light possible to prepare for winter. Removing leaves also allows water and air to get to the living plants, preventing them from suffocating. For this reason, leaves, especially tough ones that pack down and rot slowly, do not make good mulch for perennials and should be raked off perennial beds. It's too late to divide perennials, but there's still time to mulch shrubs, trees, and perennial beds with a loose organic material such as bark mulch. Do it now, and you will have one less job to worry about in the spring.
Mulches also help protect roots during winter from cold and fluctuating temperatures.
If packed around tree trunks too thick, mulch can smother the tree and cause it to die. In colder weather the roots of evergreens are frozen and unable to take up water. Winter winds may desiccate or dry them out, eventually causing them to die. Protect your evergreens by putting up a screen on the windy sides, usually the north and west. This can be as simple as erecting three wooden stakes and wrapping burlap around them.