An essay is longer, more assertive, more useful and important to others than a practice piece. An essay begs to be shared and is designed to be meaningful to a reader without added explanation. A practice, on the other hand, is almost always a bit meaningless to a reader who doesn't know the assignment or what context to put it in. When you show a practice, you almost always want to explain what you were doing.
Essays explain themselves.
Our objective is to write essays and projects that are complete and demonstrate that you can design and produce works that satisfy anyone interested in understanding your assertion and your purpose. But you will be assigned first to write practices, as one of our book's title suggests. Each is described there. They vary in length and difficulty. Their purpose is to give you experience in techniques and thought processes useful for writing compositions that communicate independently.