Answer:
It is important because the key sentence most likely contains the main idea of a paragraph and can tell a reader what the paragraph is all about.
Step-by-step explanation:
The careful observation of the key sentence is important when trying to identify the main idea of a paragraph because it gives an insight on what the supporting details in the paragraph would be.
Every main idea needs supporting details to validate them. For example, if the main idea of a paragraph is that tigers are dangerous animals then it must contain supporting details that show why a tiger is dangerous.