Dr. Staff teaches all the classes that don't have real instructors assigned at the time the schedule was put into the system.
Now, why might Dr. Staff be assigned to teach class X? Here are some common reasons:
The department has hired a new tenure-track faculty member, but the the new faculty member's information is not yet in the system.
The department uses adjunct faculty to teach many sections of course X and Dr. Staff is listed for the sections that do yet have an instructor permanently assigned. Possibly this is because the part time person who will teach the class has been identified, but the kind of contract he/she has with the institution doesn't allow their name to be put onto the course until very close to the semester. Or possibly it's because the department knows that it needs the section, but is still in the process of verifying that a current adjunct wants the course or the department may be looking for a new adjunct to teach the course.
The department uses graduate TAs for some of their sections. The TA assignments are often formally assigned at the last minute in case some of the graduate students need to change the classes they are taking for credit. Typically, the department wants the grad TA's to be able to adjust their schedule without worrying about their teaching assignment until the week before classes start.
Professor RealGuy, who was scheduled to teach course X, was awarded a sabbatical, resigned, retired, died, took a leave of absence, or took on an administrative role that requires a course release; and thus Professor RealGuy cannot teach the course. Until a new instructor is found, Dr. Staff gets the course.