Why RDF is a figure of merit for receiving antennas on the low bands, where the limit is atmospheric and man-made noise arriving from all directions rather than the receiver. It is defined as the antenna's peak gain minus its average gain taken over the whole hemisphere around and above the antenna, expressed in dB. A high RDF means the antenna rejects noise from every direction except the one you want, so the signal-to-noise ratio improves even if absolute gain is low, which is why a short Beverage or a small terminated loop can hear better than a big dipole.
Watch out Forward gain compared to gain in the reverse direction is the front-to-back ratio, which only looks at one direction behind the antenna and says nothing about noise arriving from the sides or overhead. Comparisons to isotropic or to a dipole are just gain references, dBi and dBd.
Noise comes from everywhere, so RDF compares peak gain to the average over the whole hemisphere.