Back before the internet, agents knew a lot of angles to get lower fares.
But, if the airlines figured it out they would send the agency a debit memo for the difference, so the agency would lose money.
Travel Agents made 10% commission on domestic flights, a debit memo was for the entire price of the ticket, making the debit memo usually 10 times the commission that was made in the sale of the ticket
One of the biggest tricks to get a lower price for frequent flyers was what was termed the inside out ticket
Lower priced tickets usually required a over Saturday night stay, if a business traveler was going to the same city twice, you could book both tickets with over Saturday night stays and save sometimes hundreds of dollars for the traveler
Half of ticket one would be combined with half of ticket two and the remaining halves would be combined for a second round trip ticket making both trips meet the over Saturday night requirement