Your images are subjected to whatever type of monitor and color card your audience is using. This goes back a few years when monitors were limited to 8-bit color. It's becoming more of a non issue with newer computers displaying millions of colors, but it's not uncommon to still see 8-bit laptops. The number of people with 8-bit systems decreases every year. There will come a day when web-safe color will be a thing of the past.
Some web developers feel that that day is already here.
It converts all your colors to its own fixed palette. It contains a core of 216 colors and allows 40 additional colors, which differ from platform to platform. Most prefer to work with these 216 common denominator colors when necessary. If you stick to the 216 common colors, they will be honored universally across all browsers, operating systems, and computer platforms.