What do you think is the central idea of this text (why did John Knowles feel the need to write this story?)

This is a thematic question. You can pick from quite a few themes. I think that "reflection" is a central theme in the novel; the novel is spawned by a visit back to Gene's old school, and the work hinges upon a dialogue between the past and the present, and the relation of a man to his much younger self. Gene confesses that he is still stuck in the time of World War II; his memory still has a tremendous hold on him, as evidenced by his ability to recall the goings on of fifteen years' past with such detail. The presence of memory, and its role over time, is a major theme of this book; when Gene reiterates his thoughts on the past and on the lasting impact of the events he is describing, he only increases the importance of this theme within the novel.You can check out other themes below: