Question

What makes the topic sentence so important.

Answer 1

it substantiates or supports an essay's thesis statement; it unifies the content of a paragraph and directs the order of the sentences; and it advises the reader of the subject to be discussed and how the paragraph will discuss it.

Answer 2

This is just a guess, but probably because it is the first sentence a reader sees, and so it has to be good for the reader to want to keep reading.