A web designer usually starts with a color and needs the variations of that color for hover, borders, gradients, etc. while designing/styling an element.
In order to be stable for the overall of the design, while choosing color variants, picking them from the same levels is a much better way compared to just guessing using the color picker of our image editing software.
0to255 is a simple-yet-effective web-based color finder which is specially created for web designers to make this process easier.