Internet Explorer implements a proprietary technology called Conditional Comments. While web developers frown upon technologies that aren't cross-browser supported, many CSS developers have found Conditional Comments very useful. They can have cleaner CSS in their normal stylesheets and can fix the broken rendering in IE by placing IE-only CSS inside conditional comments; this technique is even recommended by Microsoft.
Without this module, the only way to have IE conditional stylesheets was to add 37 lines of code (more if you want to add more than one stylesheet) in four horribly-difficult-to-remember function calls to your theme's template.php. Blech. Who wants that?
This module allows Drupal 7 themes to easily add conditional stylesheets to the theme's .info file.
; Set the conditional stylesheets that are processed by IE.
stylesheets-conditional[lt IE 7][all][] = ie6-and-below.css
stylesheets-conditional[IE 9][all][] = ie9.css
stylesheets-conditional[IE][print][] = ie-print.css
stylesheets-conditional[(gte IE 9)|(gt IEMobile 7)|(!IE)][all][] = modern-browsers.css