What exactly are you asking?
I'm not sure what you mean by "body content relevance" or why there would be any misunderstanding related to what function is served by content.
Body content is simply the text on the page. The topic and specific words included in that text determine what concept(s) the page is relevant for. Without body content, you cannot rank in the search engines for anything. The "function" of "body content relevance" is to determine for which search queries your page has a chance of be displayed.
This seems so straightforward and obvious, I honestly cannot think of why this would be an issue worth of questioning. If you could be more clear about what it is that you find confusing, I might be able to provide you with a more informative answer.