Why do trees go dormant during winter?

From the tree's perspective, it's to protect it from freezing and breaking. It redirects it's water and nutrients into the roots so that they are protected from the cold and available in the spring. It also reduces the water in the trunk and branches so that when the freeze does occur the water doesn't expand in the wood and burst the cells and break the branches.

From our perspective it's because of a combination of shorter daylight hours and colder temperatures. The plants somehow sense this occurring and make the necessary preperations for the above mentioned freeze.

Even evergreens go through a (mostly) dormant period where growth will cease and only enough nutrients to maitain the tree will be sent upwards.