Movies are about getting people to feel, and the tone of your script gives them an indication of how they should feel. Should they feel that it’s okay to laugh at a Nun getting killed?
An exaggerated example, but if your tone isn’t clear you risk sending the wrong message.
If you don’t present your tone clearly, you will get into all sorts of trouble. Script readers may misunderstand what type of screenplay they are reading. And wouldn’t it suck if these script readers reject your screenplay simple because of a confusing tone?
Take the example of Sunset Boulevard. It opens with the writer’s voice-over as we see him floating facedown in a pool. But originally, the movie had opened in the morgue and we heard other dead people talking to each other in voice-over. It was meant to be macabre, but after a preview audience thought they were in store for a comedy, it had to be fixed.
If your readers aren’t clear about your town, it could affect the ENTIRE script.