User preference for fixed staff positions
The option is handy for two reasons. First, it may be helpful to have a fixed reference frame for the main range of notes so that the need to consult with the octave numbers is less needed. And for beginners, most of the sheet music they encounter is going to fall in this range. The second reason is that the note range for the traditional notation treble and bass staves is almost exactly the same as the WYSIWYP fixed staff preference. This makes transitioning between the two notations easier. The following figure shows the two ranges:
The Traditional Notation tonal range for both staves starts with the bottom line of the Bass staff on G2, and ends with the top line of the Treble staff on F5, a range of 37 chromatic notes (inclusive).
The WYSIWYP fixed staves range from F2 to to F5 (39 notes), two notes more than Traditional.
Of course, WYSIWYP staves expand up and down to accommodate a score's range in the same way as Traditional staves expand with ledger lines.