If you add up the durations from left to right of any level of a note tree, you end up with the total duration of the piece or section represented at that level of the tree. Each note or rhythmic element contributes to the overall duration, and summing these durations provides a comprehensive measure of time for that specific part of the piece.
In a musical context, a note tree can represent the hierarchical structure of a composition, with durations at each level reflecting the timing of notes, chords, or phrases. This concept is useful for understanding the timing and organization of musical works.