The reason is that different computers have different display capabilities and that people use different software to view web pages.
Some computers display only a few colors while others can display millions simultaneously.
If you design pages for one level of display, and someone has a computer or browser that is below this level, your pages will not look as you intended, and may be a complete mess. This palette is supported by Netscape and Internet Explorer, among other browsers, and includes colors which are guaranteed to display correctly on Windows and Macintosh PCs using a 256 color display mode or better. Compare the results with Netscape if you have them both. This 216 color option is particularly interesting as it allows you to produce pages which will display correctly on any Windows or Macintosh computer using at least a 256 color display and one of the mainstream browsers. These 216 standard colors are more than adequate for most web pages.