[François]

Doesn't the music itself have the page number at the top?

Sometilmes, at the top or bottom, so when you use zomming, the page number is outside the view, and if you use half page landscape mode its out too.
Even when its on the visible portion of the page, it's usually too small for my small display (10.1') and my poor eyes.
So a big page number could be useful to remeber me where i am in the piece when i play.
My opinion...