All of this is actually an ancient tradition, where veterans of heavy combat would be sought out by the troops for advice during long periods of peace. A lot of the practical experience of combat gets lost or watered down when there hasn't been a war for a while and there are fewer, or no, combat veterans left in active service. The Gulf War didn't provide as much combat experience as the Vietnam war did, and Gulf War vets are spread thinly throughout the combat units. Most people in the service retire by age 55, so the last of the Vietnam combat vets are senior NCOs or officers, and there aren't many of those. Most career troops get out after 20 years of service.
Many combat veterans retire near military bases, and are available for this kind of informal training. Moreover, despite the military's best efforts, a lot of the troops (active duty and retired) hang out in bars. More useful training takes place over drinks than is realized. Whatever works.
With most of the combat veterans from the Vietnam war retired, many units are trying to keep combat experience. While the services often officially call together panels of retired generals to help pass on their experience to a new generation, the lower ranks increasingly arrange their own debriefing sessions. Divisions and battalions (and equivalent air units) hold celebrations where honored combat veterans of the unit come to speak about their experiences. Some units invite veterans, especially those who were decorated for extraordinary actions in combat, to come and speak to the troops during training exercises.