Deep Vs. Shallow
How much room does your plant need?
Well, if you want to move things around for a changing display, you don't want to deal with deep, heavy pots full of dirt when a shallow pot will do. Many annuals are succulent plants and are extremely shallow rooted -- so why waste all that space and dirt on them? Even some shrubs are happy in a relatively shallow pot. In fact there is a shallower-than-normal container called an azalea pot.
Other plants, like poppies and many perennials develop massive root systems -- some so brawny that they have been known to break pots with their expanding roots when they feel too confined. You want to give these room to spread.
A deep container will give you a chance to do some layering -- such as spring bulbs that can be planted at different depths. Tulips, for instance, go into the pot 6 inches deep and you can layer smaller bulbs and corms such as small narcissus or crocus over them for a longer spring show.