If you report someone absent on your list, and then that person subsequently gets deleted as a user (from Moodle admin), the attendance report will still report an absence from an unknown person.
That is, if you had, say, 14 students in a class, and you took attendance and listed a student as absent, then that student was deleted from Moodle, the attendance report will list 13 present and one absent (total of 14), even though the total number of students in your class is now only 13.
This is important because at the beginning of a year especially, when students are enrolling and unenrolling, the list of students you have is constantly changing. Knowing who was absent on any particular day is important for auditing purposes when the administration wants to know who actually showed up in your classes. If the totals are wrong, there is no way of knowing who showed up and who didn't.