[Coda]

It's a general art technique; it isn't restricted to just computer graphics. It's also a style decision, not a hard-and-fast rule -- certain art styles look fine with bold black outlines, and may for example vary the thickness instead of the color (art nouveau style takes this to the extreme).

It's particularly popular with computer pixel graphics because you can't get a line any thinner than one pixel, but a less-dark color gives the illusion of a thinner line, but that illusion isn't the only reason to use it.