VOICE & TONE
It's important to work out the qualities of the voice a writer creates for a text as this will help you interpret the writing much more easily.
If you want to show anger, sadness, sarcasm, hatred, this is easy to create with your own speaking voice but putting this into writing and making it subtle and realistic seeming can be much more difficult. Detecting the tone of voice in writing is important, then, because it allows you to understand what attitude the speaker has towards the subject matter. This tone of voice might also alter at different points in a piece of writing - and this is particularly so in poetry. A movement from anger to sadness can be quite comment and can be very appealing and persuasive.
The tone of the narrative voice in a story is absolutely crucial if the story is to be entertaining, believable and - very importantly - trustworthy. Trust is a key aspect of a story's narrator as it creates believability and interest.