The soil in containers warms up and dries out earlier in spring than the ground, allowing you to plant ahead of schedule. During the growing season, container-grown plants are less likely to be attacked by slugs and other ground pests. Come fall, tender plants can easily be moved indoors to overwinter. For starters, they can jazz up dull areas. Notice how the potted plants in a garden invigorate a scene that would otherwise have a large bare spot.
Containers can also serve as focal points, such as the one decorating the corner in the photo above.
Just as important, containers highlight plants by bringing them closer to eye level and allowing trailing plants to cascade down the sides. They've all been used as plant containers. Anything that will hold soil can be used as a garden container. Additionally, a good container should resist rot and corrosion. Unless you're growing water plants, the container should also have a drainage hole.