Description has many uses. Of course it helps the reader see the setting and the scene, but there’s more to it than that. As with most things writing, when you get into it, it’s complicated.
The description needs to fit the voice of the narrator, the overall tone of the book, and the emotion and tension of the scene, and it needs to add something of its own.
The most important part about writing description is, do not write every little detail. Pick out a few important points and describe them; allow the reader to fill in the details on the rest. For example, rather than going into full detail on the entire house, describe the way the dark blue paint peels away from the weather-worn oak beneath. That gives a broad sense of the entire house, and it adds a little atmosphere of its own without weighing the entire scene down and slowing the pacing.
Remember to pick out the details that the narrator would see, the things that would stand out in their mind. For example, one character may not notice the peeling paint at all, but they could be incredibly creeped out by the pitch-black bark on the stunted, thorny tree which keeps clawing at the broken window. That gives some insight into the character at hand, their focus and their feelings on the situation. It also gives you some room to add in a bit of atmosphere.
There are moments when you need to give a quick look at something, and others when you need to give the reader a real look at the surroundings. You want to be briefer when the focus of the scene is on the characters, their emotions, their words. That’s when you don’t want the setting to detract from that focus. Instead, you want things short, brief, and carefully chosen to add impact to the words and emotions. For example, if Lisa is arguing with Arron about how he hasn’t moved on from Cindy, then she may notice that the vase contains a crisp-white lily with deep pink stripes running down the centre of each petal. Cindy’s favourite flower, there as the centre focus of the entire room.
At other times, the setting and surrounds are the focus. Maybe Meryl is in awe at the new spaceship she’s just been dragged to by her alien kidnappers; she never knew such a thing existed up until that moment. She’s entirely absorbed in the way the soft blue light almost seems to trickle down the dark grey pipes which stand exposed against the blood red walls. Somehow, the aliens have made the necessary functions of the ship something artistic. Even the dull blue seats in the cafeteria area add something to the overall beauty of the ship.
You need to consider where the focal points of the characters lie and where the focus of the scene is. If the focus of the scene and characters is on the characters themselves, then keep the descriptions short. They need to add extra weight to the dialogue and emotions of the characters, nothing more. On the other hand, when the focus is the setting, the description needs to display the focus of the narrator and fall on the key elements which display the surroundings to their best, while still leaving gaps for the reader to fill in for themselves.
Don’t give away everything, describe important little bits. Show the reader the details that the narrator picks out and notices, then allow the rest to form in the reader’s mind.