From Leonard Ornstein's Tarantelle (1963)
Unlike pedal markings, these are above the bottom staff. I feel like it could be phrasing, but that's normally indicated by the beaming of the notes. What do these groupings mean?
It is phrasing. It's a kind of regrouping of the beats in the left hand, from 4 beats in the bar to 3. It's much more visual using the brackets than just using phrase marks.