If your track can accommodate 20 Amps, why do you advise to use only 80% of capacity?

The track can fully handle 20 Amps, but because of field installation conditions such as the quality of existing building wiring, track distance run, number of connections, etc., it is common practice to de-rate the track by 20%.  An additional benefit of using a conservative load factor is that you have the opportunity to add new fixtures to your track in the future if needed.