Predictability is important to a reader, just as knowing a ski trail is important to a skier. Readers and skiers are comfortable when they know what is coming and when what comes is what they expect.
Parallel construction (also called “parallel structure”) fosters predictability and meets the reader’s expectations. As explained by Wikipedia, parallel construction gives two or more parts of a sentence a similar form, imparting a definite pattern to the whole. The continuation of the pattern fulfills the reader’s expectations.
