Background: Polyphonic voicing
Polyphonic voicing allows you to resolve a number of situations impossible to score properly otherwise:
Notes starting at the same position, but with different lengths. Without polyphonic voicing you get unnecessary amounts of ties.
Vocal scoring and similar. Without polyphonic voicing, all notes starting at the same position are considered parts of a chord. With polyphonic voicing you can give each voice a stem direction, you can have individual rest handling for each voice, etc.
Complicated piano systems. Without polyphonic voicing, you have to resort to a fixed split note setting to decide which notes go on which clef. With polyphonic voicing, the split point can be “floating”. The program can even automatically put a bass line on the lower clef for you.