What is the effect of antithesis?
It can be seen as part of a process involving thesis and synthesis.
In literature it is a setting or character or mood etc which is set against what has gone before. Therefore it not only highlights what is being described in itself but also that which has gone before. From this the two forms of expression or description can be used to view or analyse a situation from more than one perspective.
An example would be from David Copperfield. Characters such as Betsy Trotwood and Mr Dick were the antithesis of Mr Murdstone or Mr Creakle. They showed David kindness and love whereas the latter were brutal in their treatment of the eponymous hero.
Therefore we can juxtapose the two to look at human relationships from more than one perspective. Dickens did that in all his novels.