As hinted by the name, comparative reasoning establishes the importance of something by comparing it with another thing that it holds some similarity with. This could be by comparing different levels of a common quality, comparing reality with ideals or comparing words and actions against mores and values. For example:
Tammy and Jeff have cut their choices down to two cars, with Tammy favoring one and Jeff the other. Tammy’s choice has five seats while Jeff’s has seven. Jeff’s choice is bigger and so has worse fuel efficiency but better off-road capability. Tammy’s choice is also cheaper, and value is an issue. Tammy and Jeff are planning on having no more than three kids anyway, so Tammy’s choice wins as the one best fitting their needs.
It is difficult to judge things on a standalone basis. Introducing another factor makes the process of judgment so much easier. There is a bit of variability involved in formulating your comparative argument. If you want whatever is being discussed to look good, you compare it to a weaker example. If you want it to look bad, you compare it to a stronger one.