Parallelism

Parallelism requires that elements in a sentence that are in series or that are being compared or contrasted be expressed in the same grammatical and syntactical form.
Let’s look at parallelism in series construction: it’s only a hop, skip, and jump away. All three items are nouns, and this makes the sentence flow. Contrast the effect of violating parallelism: it’s only a hop, skip, and jumping away. The switch from a simple noun to a gerund phrase sounds awkward and it confuses the meaning. Consider, again, the difference:
Government of the people, by the people, for the people.

Perfect parallelism such as this is a masterful rhetorical style. Note that each item in the series is a prepositional phrase. In contrast to:
Government of the people, by the people, and with the needs of the people foremost.
Changing the 3rd item of the series from one prepositional phrase to two alters the rhythm and flow of the phrase, significantly damaging its impact. Sentences that violate parallelism disturb the reader like an off-key note disturbs a listener; both jolt and distract. The smooth flow of meaning is stopped until the reader can remove the blockage.