Tribulation gives the leaders an excuse to carefully control everyone and create "Purity of the Race" (ch 6).
Tribulation was the holocaust or apocalypse-like incident that wiped out a large part of the population and caused them to start over. The idea of Tribulation is based on the expulsion from Eden as a result of Eve’s sin.
Tribulation had been another such punishment, but the greatest of all: it must, when it struck, have been like a combination of all these disasters. (ch 4)
The people are so caught up in Biblical imagery that they refuse to accept anything that is an anomaly, including people with deviations from the norm. They feel that they are preventing another Tribulation by doing so.
It is easy to control people who are afraid they will offend God if they do anything other than what they are told.
Not everyone believes that Tribulation was a sign to bring humans into a strict God’s-image society. When David and the other Telepaths enter the Fringes, they hear a different point of view from the philosophical man who takes them prisoner.
'Some day,' he proclaimed, 'something is going to steady down out of all this. It'll be new, and new kinds of plants mean new creatures. Tribulation was a shake-up to give us a new start.' (ch 14)
Of course, in the end the Telepaths meet the Sealanders, a group of people who are also telepathic. The Sealanders tell them that their people are living in history, and they are the future.