Why is mastery over sentence structure important

Sentence structure is pretty important. Why? For one, so you can understand the exact meaning that the person wants to portray. Also, so you can communicate more easily and effectively. If I said: "My sister bought a shirt from the department store that costs three dollars" What? The department store costs three dollars? No, the shirt did. What you just witnessed was a misplaced modifier. Other grammar slips are quite amusing, but none of it would have never happened if you had only used proper sentence structure.
It also helps with diagramming.