The two staves used in piano music (tend to) represent the two voices of the left and right hands. In fact, notation for any pitched instrument uses a clef suited to its register, or interval of notes it is capable of playing, so that they are centered on the five-line staff. It wouldn't make sense to notate music for every instrument on a single expanded staff, where each one only utilizes five lines.
This ends up being particularly helpful when reading scores, because you can see at a glance where each instrument is playing in their register. Who's playing the melody? Just look for the notes that are highest up on the staff. Notes that are high in an instrument's register tend to have more high frequency partials and stand out in the mix.
This ends up being particularly helpful when reading scores, because you can see at a glance where each instrument is playing in their register. Who's playing the melody? Just look for the notes that are highest up on the staff. Notes that are high in an instrument's register tend to have more high frequency partials and stand out in the mix.