Every piece of sheet music you open (see Viewing your sheet music) is kept as a local copy afterward, so it's still available with no signal — useful backstage, but it does use storage over time.
From More > App settings, the Clear cached scores row shows how much space these local copies are currently using. Tap it to remove them all at once. This only clears your phone's offline copies — nothing is deleted from your band's actual sheet music library, and the next time you open a piece it simply downloads fresh again.