Railroad tracks, fence posts, and gymnastics bars. What do all these things have in common? You got it: they're parallel.
Got a series of nouns, verbs, or adjectives? They need to be structured the same way. Ditto for clauses or items being compared and contrasted. If two of your three items are fragments, the third better also be a fragment. If you start two bullet points with adjectives, each bullet point needs to start with an adjective. In short, your sentences need to be balanced.
Just like breakfast.