You can use it to make things on your site semi-transparent.
I've one button left on my site that still uses the Opacity tag (bottom, center "Abigail Dreams").
I use to code the entire site with that, making almost everything semi-transparent in one way or the other, even the iFrames. Now I just use a darker shade of image for the backgrounds on things...
<div> - putting your mind in a box since 1997