Why is it important to identify the purpose of a piece of writing?

To find one definite interpretation.

Explanation:

The purpose of writing of a text may be one but it can be interpreted in a variety of ways which may or may not have been intended in he beginning by the author of the text.

To do away with this sort of confusion it is important for us to understand the authorial intent with the text so as to work towards an interpretation that may indeed have been intended and not interpret as the mind and its own bias takes the reader ignoring what the author meant.