Parallel structure is a convention in literature which requires consistency in use of style. The intention is to organize information into context. The basic principle is that mixed expressions reduce the effectiveness of content, so it's best to use the same forms of expression. In effect, it's a form of syntax created for better cohesion and continuity.
Examples of Parallel Structure:
|'Reading maketh a full man, conference a ready man, and writing an exact man' Francis Bacon, Essays, 1625.|
'Ask not what your country can do for you; ask what you can do for your country.' John F Kennedy, speech.