footprint is a summary of (cell width/heigth) plus (cell pin location and geometry) plus (cell metal polygons geometry).
If two cells have the same width/heigth, the same pin geometry and all other metal polygons are the same, it means these two cells have equivalent footprint.
It is needed, when you do want to replace one cell for another (timing optimization or other reason), but do not want to touch existing placement and/or existing routing.
It is obsolete (still supported) feature. Usually, the tools derive such info from LEF or Milkyway (not from .lib).