Say Bob and Rob have a written contract.
Bob becomes unhappy with what Rob does and sues him for breach of contract. Specifically, Bob applies for a summary judgment because, in his view, the breach is so clear so that no hearing/trial is needed.
Rob opposes judging the case that way. This is what "defence to summary judgment" is — Rob wants a hearing. He says that the contract was modified orally, and in the light of that modification there is now no breach as Bob alleges, so we need a hearing so that his right to natural justice is not breached.
The result is that the court has to hold a hearing — instead of deciding the case on the papers. Rob can effectively slow down and tamper with justice by claiming that he and Bob orally agreed to whatever.
If the oral modification was not allowed, Rob would not be able to stop the summary judgment by abusing his right to natural justice.