I think sometimes we have to choose answer choice that makes most sense after process of elimination even though that answer choice does not really capture the main purpose of the paragraph. For example, paragraph illustrates about principle A and talk about the relationship between that principle and subject X. So, I naturally assume the primary purpose of paragraph is to show the relationship between principle and subject but sometimes there's no answer choice corresponding to that, and instead correct answer choice could be something like "Illustrate about principle A".

Really a good question