Descriptive language is used in literature to create a picture in the reader's mind. When an author writes a story, the story has to come to life for the reader to be able to understand where the story is happening, or what the character is like. In the exposition or beginning of the story, the reader is given details about both so that they know when and where the story is set and what kind of person is living in this story. Think of a story like "Charles" by Shirley Jackson, and then put the story in a dark island setting like "The Most Dangerous Game", or change the character to a college freshman. The story would not work because it isn't meant to be a dark, dangerous setting, nor an older character but a little boy. As the author gives you more descriptive details about the characters or setting, a clearer picture emerges in your mind as you read the story and understand more clearly the characters or setting or tone.
Descriptive language is of course vital in order to build up character, setting and tone. It is going to be really hard to answer this question without refering to a specific example, so let me quote a section from the beginning of "Games at Twilight" by Anita Desai. Consider what the descriptions of heat add to the passage and help us to empathise with the children:
It was still too hot to play outdoors. They had had their tea, they had been washed and had their hair brushed, and after the long day of confinement in the house that was not cool but at least a protection from the sun, the children strained to get out. Their faces were red and bloated with the effort, but their mother would not open the door, everything was still curtained and shuttered in a way that stifled the children, made them feel that their lungs were stuffed with cotton wool and their noses with dust and if they didn't burst out into the light and see the sun and feel the air, they would choke.
Note how the images of "bloated red faces" and the description of the stifling heat and stillness, stuffed lungs and noses and lastly a choking sensation of entrapment as the children desire to get out into the outside world really helps convey a sense of the claustraphobia and helps us to understand how they are feeling and their eagerness to get out. Here we can see therefore how good descriptive writing allows us as readers to ultimately imagine the scene much better, and become aware of the feelings of the children whose characters dominate this short story.