I use the square end of the one on the left to drive nails into sash cord knot holes. I use the one on the right to seat the final nail on zinc weatherstripping after the sashes are locked together. This usually ensures precise operation.
Choke up on the handle and use the flat side to start a brad nail. Then, choke back down and use and round side to drive it home.
Nail into center of knot so as not to puncture any rope fibers. Angle nail downward so upward pressure won’t dislodge nail. If hole is deep and nail has the possibility of striking glass, angle nail away from glass.