A lot of our teachers use assignments but don't use the gradebook because they have their own grade record-keeping methods. They would like to be able to change the basic status of assignments to something like "completed" or tag it as "missing" without having to add it to the gradebook.
Often a teacher will have an assignment they want students to complete and submit but it is not graded. An example would be practice programming exercises in computer science. In order for a teacher to be able to mark the assignment as "missing", it has to be in the gradebook. This "missing" notation is one of the ways we can signal to students, parents, and counselors that the student isn't completing assignments even if their grade isn't being affected by those missing assignments. In addition, since we currently have to add the assignment to the gradebook, the glaring "Graded: 0" makes it feel like the teacher just isn't doing their grading.