When does one use an extra stave for piano music?

Elaine Gould recommends the use of additional staves "if multiple parts are otherwise hard to read" (Behind Bars, p. 332). That is clearly the case here, since with only one bass stave, chords would collide with the pauses and tuplet braces.

1. What does it mean when sheet music has 3 staffs?
2. What is the purpose of the grand staff?
3. Why does piano sheet music have two treble clefs?

What does it mean when sheet music has 3 staffs?

Sheet music with 3 staves; one grand staff (treble and bass) and one more, usually treble, indicates another instrument/voice. The piano then often has only the accompaniment written out (based on the chords) and the melody is meant to be sung.

What is the purpose of the grand staff?

The Grand Staff is used for piano music, and when the barlines are drawn vertically to break the staves up into measures of a certain length, the lines are drawn through both staves and the space in between them.

Why does piano sheet music have two treble clefs?

The two staffs on a piano grand staff are primarily used to differentiate between your left and right hands. Sometimes, the left hand will need to play notes in the treble clef range, and sometimes the right hand will be playing notes in the bass clef range.