A thesaurus provides alternative words that are more suited to the image/feeling/mood a writer wants to convey.
It was a dark and stormy night. ---- Cliché description.
Syn. for dark: aphotic, atramentous, black, blackish, caliginous, clouded, cloudy, crepuscular, darkened, dim, dingy, drab, dull, dun, dusk, dusky, faint, foggy, gloomy, grimy, ill-lighted, indistinct, inky, lightless, lurid, misty, murky, nebulous, obfuscous, obscure, opaque, overcast, pitch-black, pitch-dark, pitchy, rayless, shaded, shadowy, shady, somber, sooty, stygian, sunless, tenebrous, unlighted, unlit, vague
Stormy:
bitter, blowy, blustering, blustery, boisterous, cold, coming down, damp, dirty, foul, frigid, furious, gusty, howling, menacing, murky, pouring, raging, raining cats and dogs, rainy, riproaring, roaring, savage, squally, stormful, storming, tempestuous, threatening, torrid, turbulent, violent,
So you could creatively shift the mood of your sentence by writing:
In the dead of darkness, the air was tenebrous and turbulent.
The thesaurus helps the writer find just the right words. It was my favorite book to read as a kid.