It's easy to fail to realise the importance of where a story occurs and, even more importantly, how effective the author has been at creating a believable setting.
Settings are always imaginary and yet we are so easily drawn into believing they are real. Sometimes authors use real place names and apparently real seeming details - but again, this is a literary device used to trick the reader and absorb them into the story's plot.
First and foremost a setting is used to create a sense of place - one that is believable, interesting and useful to the plot. Other times the setting can be used to help create mood or even to give an idea about a character. Your own bedroom, for example, well described, could be very revealing of you as a person.
In your essays, always try to reserve space to discuss in detail and with subtlety how an author's use of setting works and contributes effectively to the story.
When a setting is being used to reflect the mood of a character, this is often referred to as a pathetic fallacy or as a mental landscape.