Some trees can be planted in containers provided the container gives enough room for their roots to grow. This leaves all the roots intact and helps reduce plant shock when transplanted. Container grown trees are much lighter and easier to transport and plant than their field grown relatives. Since the containers can be moved it is easier to protect them from the weather and keep them available longer into the season.
Some trees can be easily cared for in containers. This would let you use them on a patio, indoors, or transport them around the yard as your gardening project evolves.