When contrasted with Action and Dialogue, Description can be a yawner opening for a book. Who wants to read paragraphs about the weather, the view of the mountains, the sound of a river, or the pattern of wallpaper? None of this tells us what the story is about, who is in it, or what he or she wants.
However, well-crafted Description can make or break a book. It provides a sense of place (the Setting for the story), and can reveal how a character feels about his surroundings (Scarlett's love of Terra in Gone With The Wind), obstacles it represents (such as conquering the violent sea in the 2000 film, The Perfect Storm), or impart a sense of menace, dread, or hope (the post-apocalyptic worlds of Blade Runner or The Hunger Games.
When traveling, I take pictures of things I might want to describe in a future scene. Of course, I can't capture the smell of green grass or the taste on the back of my tongue if I picked and chewed on a tall weed, but the visual can spark memories of those things.
By remembering and describing in detail, authors make the character relatable. Readers have also picked and chewed on those tall weeds. Perhaps they've tried to use a hollow stem to make a whistle. (My cousins did this all the time, but I could never master it.)
Many readers have played in an old barn. They remember the smell of hay or hot motor oil in dirt, the sound of creaking boards in the hot sun, and the way the light filters in through gaps between the siding. Our job as storytellers is to remind them of those things—through the eyes of our characters.
While Action and Dialogue may hook the reader and keep the pages turning, description is what lends beauty and depth.