There are many benefits of starting with a dormant plant. This type of vegetation is less susceptible to outside factors, like frost or sunlight. This is because the plant has not yet started to grow for that season.
Dormant shrubs and perennials are able to be planted sooner due to their cold hardiness. In Michigan, it is ok to start planting bare root blueberries as soon as the ground is soft enough to dig a hole. The benefit of starting blueberries in early spring is the plant has more time to become established before its first winter.
Some trees and shrubs do better than others when starting from dormancy. Fruit trees and other young trees tend to do very well when started in the spring from dormancy. Do your research before buying any bare root tree, as there is a certain timeframe and method for planting bare root trees.