Discussing the structure of a text is important to gaining high marks.
Structure refers to the way topics and ideas in a piece of writing are revealed in a sequence by the writer. Releasing ideas in a sequence can help make the writing clearer (as in writing to explain), it can develop tension and interest (as in a narrative or story), it can make the writing more persuasive (as in writing to argue).
Structure can be said to 'shape meaning' and is clearly one of the most important aspects of writing; certainly, if you fail to take account structure, you'll lose marks. It's that simple!
You can work out the structure for a text by making a note of the ideas or topics that the writer explores as the text proceeds. Always consider the effect and purpose of this sequence to recognise to recognise how the whole meaning of the text is slowly being shaped.
It is usually important, too, to discuss a text by working through it from beginning to end - if you don't you are appearing to ignore the writer's well-thought-out structure!