At night, when the horizon is not clearly visible and pilots have almost no visual reference until they are on short final. In bad weather, especially when there are low clouds, thunderstorms, fog, etc. Or during a snow storm, when the flightdeck is covered and made slippery by snow.
The photograph in this post is one of the few you can find online showing an aircraft landing on a snow-covered flattop’s deck.
It was taken on Jan. 21, 1987, and it depicts an A-6E Intruder of Attack Squadron (VA) 52 on final approach for recovery on the snow-covered flight deck of USS Carl Vinson (CVN 70) operating in the Bering Sea.
Image credit: U.S. Navy