Early method of making the script run one way in IE and another way in a different browser, also had some predefined variables for handling versions and whatnot. You may remember the same feature but for HTML: <!--[if IE 6]>

Obviously modern HTML tags with fallbacks, @supports in CSS, and proper feature detection instead of hardcoding are the better approach nowadays since everyone finally agreed to follow the standards reasonably closely and you don't need version specific hacks rather just checks that something is supported.

Okay that definitely makes more sense. I use those IE conditional comments to this day in email templates (for Outlook).

You know you have a problem when you need to alter web standards in order to let programmers get around your alterations of web standards.

As a new HTML coder back in the day these conditional comments were really odd to me. Thankfully we don't have to deal with that much anymore although IE 11 is still out there, lurking.

C supports conditional compilation, where it's great for supporting multiple architectures and operating systems.

well sometimes people want to selectively run code in a dev instead of a prod environment. I wouldn't say it's a bad idea, it is a possibility we sometimes need that it would be preferable that we did not need and we should try to get away from needing quickly if we find ourselves in that situation.