Just as Socrates felt that it was necessary to create a tension in the mind so that individuals could rise from the bondage of myths and half truths to the unfettered realm of creative analysis and objective appraisal, so must we see the need for nonviolent gadflies to create the kind of tension in society that will help men rise from the dark depths of prejudice. Why does King use this allusion?

Reason for Martin Luther King need for constructive non violence

Explanation:

As per Martin Luther King - There is a type of constructive non violent tension that is necessary for growth.

King gave example of Socrate's stated necessity of tension for personal growth. He emphasised on need of constructive non violent tension that is necessary for growth.  As it helps men to rise from dark depths of prejudice & racism - for understanding & brotherhood.

The purpose of this action of allusion is - creation of crisis packed situation, that will inevitably open the door to negotiation. This mutual negotiation in society is likely to create base for ending discrimination & enhancing social harmony.

the words “strangest” and “unusual” suggest that the narrator is about to tell the story of an odd or puzzling group of people.