Question

why is an interpretation of a source text important?

Answer 1

It gives a different perspective on the original work's main ideas.  

It gives clues as to what is most important about the work.

Answer 2

If you use a source text as it is, you are basically copying or you are plagiarizing.

However, if you interpret the source work, you are adding your understanding to it, hopefully giving it some dimension and depth because you have looked further into and shared what you found yourself.

Is also gives a different perceptive on the original works main ideas.