I think this question can best be answered by refering to how the different relationships that are played out for us in this novel support and relate to one of the key themes of the text, which is that of marriage. Your question refers to the way in which the complex plot shows "contrast and assessment," and one of the ways in which this is achived is through the juxtaposition of three courtship and marriage plots and the way in which they are compared and contrasted.
Consider the similarities between the courtships and marriages of the Casaubons and the Lydgates. These marriages, although very different in some ways, both indicate the wya in which the expectations and illusions that we often have during courtship are destroyed by the reality of married existence. All four of these characters identify the qualities that they are looking for in their future partner, even though this is shown to be fantasy and these projections are actually irreconcilable with the projections of the other. Thus it is that Dorothea sees marriage with Casaubon as an entry point into the world of male knowledge, whereas Casaubon looks for nothing more than a reader and a nurse and somebody to help organise his papers. Rosamond desires prestige and wealth and thinks marriage with Lydgate would offer her this. Lydgate believes Rosamond would be able to support him as his wife after his days of toil in scientific research. The reality of both marriages is incredibly different from what these people were led to believe.
These two couples are in turn juxtaposed with what is suggested as being a much happier and realistic example of marriage. Mary Garth has no misconceptions about her desires and her own importance. She thinks that "things were not likely to be arranged for her peculiar satisfaction." In spite of her feelings towards Fred, she waits until he is able to display the kind of independence and industrious nature that she knows he needs to develop and mature. In the character of Mary Garth, therefore, Eliot presents us with how self-awareness, practical common sense and a true understanding of your position in the world are invaluable qualities when it comes to choosing who to marry. The structure of the novel implicitly compares these three relationships and finds two wanting.