It comes up when a creature tries to see the cloak-wearer and the outcome of that effort is uncertain. Because I run vision as pretty much binary, this only comes up when the cloak-wearer is on the very edge of the observing creature's visibility. So, for example, if the cloak-wearer is about two miles away on a clear, brightly-lit day with an unobstructed, unobscured view from the observing creature, or if the observing creature is on a mountain about 40 miles away under similar conditions, or if the cloak-wearer is about a mile away in an area of light rain (which does not obscure), or if both cloak-wearer and observer are underwater at a distance of exactly 60 feet, then if the question arises of whether the observing creature can see the cloak-wearer I set a DC depending on the conditions that the observing creature rolls against at disadvantage.