Web developers often need to ensure their sites render correctly (or degrade gracefully) on older machines. While tools like Internet Explorer 11 "Enterprise Mode" or BrowserStack exist, having a local portable instance of IE8 or IE9 offers a quick way to debug specific rendering bugs without rebooting into a virtual machine.
: Using older versions of Internet Explorer is highly discouraged for general browsing due to severe security vulnerabilities that are no longer patched. Lifecycle FAQ - Internet Explorer and Microsoft Edge