I’m a member of three critique groups, so I spend a lot of time reading other writers’ work. I often find awkward sentences that violate the rules of parallel structure.
Why is it important?
Parallel structure makes your sentences graceful. People may not be able to tell you why your writing is awkward, but they feel it. You may not know how to describe what is wrong with your dance partner’s tango steps, but you feel it when they* step on your foot.