Setting of the story is very important for a writer and it needs to be well described. Every story has to take place somewhere, and writing about it cannot be ignored in your story. No doubt, setting is an integral part of your story.
Why do we need to describe about a setting?
A proper description of the setting where the story is taking place helps the reader visualize the place and how it must be taking place there, while reading your story. If the description was not given the reader won't be able to get connected with the story.
It makes the reader the story is more believable. You must have read reviews which say the story was more realistic. The author would have put enough details for the reader to feel it realistic and believable. Though a well described setting is a pleasure to read, too much description can be a boring too. Care to be taken not to write too much to an extend that the reader gets bored, instead of getting interested in your story and turning the page.
When you are writing a setting you are 'showing' it and not telling it. You must have heard many advices from expert writers that while writing a story 'showing' is more important, and not 'telling' it, so that the reader can enjoy the story better.
If you are in the process of writing a story don't forget to focus on the story setting of the book you are reading. It will give you better ideas for describing your setting.