They implemented it with ie5 and saw it as a good thing to optimise sites for IE.
Conditional comments have certain advantages over scripting methods of browser detection.
When a downlevel browser encounters a downlevel-hidden conditional comment, the browser skips over the HTML inside the comment, and the content elements are not parsed, downloaded, or rendered. This saves client machine resources.
Conditional comments do not require scripting and DHTML, and when no scripting is used in a Web page, no scripting engine needs to be loaded. Conditional comments are processed during the downloading and parsing phase, so only the content that is targeted for the browser is actually downloaded. Conditional comments can be combined freely with other browser detection techniques.
- Separate code from detection logic.
Using conditional comments, script logic can be separated into smaller and simpler segments of code, which are easier to maintain and understand. Plus, code segments are loaded only by the browser version for which they were intended.
Conditional comments have been around since Internet Explorer 5, but their use is not restricted to Internet Explorer alone. Conditional comments can be used to customize content delivered to browsers that support conditional comments and those that do not.
more info: http://msdn.microsoft.com/en-us/library/ms537512(v=vs.85).aspx
