What is parallel grammatical construction? why is it important?

In grammar, Parallelism, also known as parallel structure or parallel construction, is a balance within one or more sentence of similar phrases or clauses that have the same grammatical structure

Similar headings throughout an outline should be constructed in the same way.

Explanation:

When writing an outline, like headings are easier to understand when written in parallel grammatical construction. The grammatical construction used in one heading should be used in all other similar headings throughout the outline.