- Any desired font could be embedded into the flash file
- No images were required
- The same flash file would be used through the site, thus cachable and more efficient than images for bandwidth
- Any copy updates to the HTML were reflected immediately as the JS simply fed the text into the SWF
- Most people had flash, and if they didn't, the text in question would still be rendered as plain ol' text, just not with a custom font
- Predates jQuery
For the truly curious, you can find the sifr generator here.
Sure you took a significant performance ding in 2005 for polluting your site with random SWFs but ended up with a site with custom fonts. It was an elegant application to a stupid solution.
Fast forward to 2016: Flash is dead and websites now ask for permission to run flash. This is an unforeseen problem as it doesn't fail gracefully in modern browsers and in the case of FireFox leaves black squares on the web site, waiting for the user to allow flash to run.
How to fix
Fortunately the quick fix is easy, remove the sIFR.js. Look for sIFR.replace in your JS to see all the targeted text that's being replaced with an SWF, to write out new CSS rules to format. Hunting down the fonts can be a bit of a chore but hopefully the swfs are named after the font. If you can't find webfont versions, just hit the usual grounds of Google Fonts or Font squirrel to find "like" font replacements. In 2016, its unlikely many sites are still up using sIFR.js. Good luck out there!