It’s important because trees that don’t drop their leaves in the winter, such as Junipers, Pines, etc., are still active and lose moisture through their needles. These trees, without supplement water, will draw from their built-in moisture reserves which may cause them drought stress.
Remember, roots remain active during the winter. Trees, shrubs and perennials need moisture to remain healthy through their dormancy. Healthy roots make for more robust plants next Spring. It’s okay to water even if it freezes because a moist freeze does not cause as much harm to the plant as a dry freeze.
Don’t forget to check plants in your containers. They too will need some water.
One last thing, remember to drain your hoses and disconnect them from the hose bibs.