Readability is not a matter of taste. It is a ratio between the relative luminance of two colors, running from 1 to 21, and there are numbers you are supposed to clear: 4.5 to 1 for body text, 3 to 1 for large text and interface components.
Almost nobody checks, because checking means sampling two colors, opening a website, pasting them in, and repeating that for every piece of text in the design. So people rely on their eyes, in a dim room, on a good monitor, which is exactly the situation where a failing combination looks fine.
Check Contrast goes through the text layers in the open document in one pass and reports the ratio for each, with a clear pass or fail. Where something fails, you now know which layer and by how much, instead of a vague sense that the gray might be too light.
Two things surprise most people the first time. Yellow and orange are far brighter than they feel, so white text on them almost never passes. And a light gray that looks elegant on a desk monitor frequently fails outright on a phone in daylight.
