They offer the opportunity to give strong similarities to something or subject, and contrasting ones giving sharp differences. There are two ways of arranging the subjects; the block and point to point arrangements. They are basically a platform to support personal desires with comparisons to others.
In the block arrangement paragraph A will be followed by B with similarities the difference being the uniqueness. For instance if you are writing on comparisons over air and rail transport; In the first paragraph you give a fact that will be replica of another fact in the second paragraph, in the same order. You decide to go point by point, the issues stand distinguished in their own epigraphs and sub paragraphs with the same criteria. In order to get the best outcome, let the sequence of ideas be organized and well prioritized.