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.
they are dormant in the winter
Deciduous tress do go into dormancy during winter months. Deciduous trees will start to lose their leaves in autumn. This process is called abscission.
because they go dormant or go to sleep
How often do you water trees in the winter?
The leaves of some trees change color in the autumn, as the chlorophyll is not renewed in them. These trees (deciduous) go dormant during the winter.
Because - during winter months trees go into a 'dormant' state - Most lose their leaves, and sap production is vastly reduced. Sap production resumes in spring - so it's better to tap the trees then.
Trees hibernate or go dormant during harsh weather when the temperatures are colder and there is less sunlight. Depending on in which hemisphere the trees live, they could go dormant during October to April in the Northern Hemisphere, or April to October in the Southern Hemisphere.
During the summer in the northern hemisphere (where most of the vegetation is) trees and plants grow, removing carbon dioxide from the atmosphere. This lowers levels in the atmosphere. During winter, when the trees and vegetation are dormant, levels rise again.
There are many species of cockroach that have a dormant period during the winter. When the temperature at night drops below freezing, most cockroaches go dormant and will die after a week.
Hornets go dormant during the winter months, or when the temperatures go below freezing. Most of them do not survive and those that do build new nests in the spring.
Daisies do not die in the winter, they only go dormant, which means the more or less hibernate during the winter, returning in the spring
Frogs don't sleep during the winter. They go into hibernation, which means they are dormant for long periods of time.