Every part of a design stack wants colour in a different notation. CSS takes hex, a design file shows RGB, a gradient generator thinks in HSL, and a brand guideline may specify something else again. Converting by hand is error-prone and the mistakes are subtle, because a value that is slightly wrong still looks plausible until it sits next to the correct one. These tools convert between the formats exactly and show every notation at once.

HSL is worth learning even if you write hex everywhere else, because it is the only common format where the numbers mean something visual: hue is the colour, saturation is how vivid it is, lightness is how bright. That makes building a consistent palette much easier, since you can hold hue steady and vary lightness to get shades that genuinely belong together. Check the result with the contrast checker before shipping, so text stays readable.