Each relic had an associated indulgence that reduced the time a sinner had to spend in purgatory by days or years. Added together, the relics in the church collection could bring about a reduction of precisely 1,902,202 years and 270 days in purgatory.
That may have been the origin of the "must see" exhibition.
Oh, and they then used the money collected by people come to see the exhibit to fund bridges, dikes, schools, hospitals, and cathedrals. People were willing to pay for what they imagined and the authorities used that to pay for what was real. That's an interesting governance model and may have actually been more sustainable than the one we have now.