Why the black area on projector screen?

[Boeriemore]

I have to position a projector according to the position of my 16:9 screen but am confused about the
220 mm black section above the white viewing area.
Does this ever serve a purpose or can it be kept rolled up within the casing?

 

[TimbaLand]

 

[chrisc]

 

[SphereCDI]