I have sometimes used slightly grey and used "full" black for small emphasis (when the bold version available for the font in use is a just too much or you want a level in between). On pages where other colours are used for highlighting things (links, call-outs, warnings, ...) it can also help them not look a bit more subtle than you want agaist the hard black.

You have to be very careful not to rely on it though: as you mention lighting and protection conditions or just plain bad monitors can kill the more subtle difference completely. And, more importantly, those with sight deficiencies might miss it even in ideal circumstances.