In the 80s/90s, travel-agents had a terminal that would allow them to search for and book travel much as one would do with Google Flights/Expedia/Travelocity today. Access to those networks cost real money, so travel agents became middlemen.

Many agents did curate trips (or sell packages), too. In the best case, if your trip went sideways, they could also assist in rebooking/other options.