Questions that ask about the “function” of a detail, sentence or paragraph is a Logical Structure question. These frequently appear on the GMAT — always look for the logical keywords that tell you where the author is taking the discussion.
Authors organize their ideas in paragraphs, and each paragraph has a mini-purpose. Why, otherwise, would the author write it? Try to get a sense of the function of EACH paragraph as you read by looking for the keywords. Put the “function” in your own words and write it down!
Since keywords and phrases are ALL you have to go on to extrapolate the structure of the passage and the author’s intentions, practice pulling out the key phrases in this passage. Then scroll down, comparing them to those the author identified.
This blog is for GRE passages, but they are the same in terms of form and content to the GMAT. Notice the list of “Function” verbs here. Copy them down and try to “assign” an infinitive verb to each paragraph. What is author DOING with each paragraph?
Just like every paragraph has a function, each sentence within each paragraph has a function. Start with the overall function of the paragraph, then ask: how does this detail relate to the paragraph’s overall function? Is it aiding the main idea? Qualifying it? Making a concession?