The goal of having different versions of Internet Explorer is to check the look of your website when being browsed.
It installs versions from 1.0 to 8.0 passing by the rest: 1.5, 2.01, 3.0, 3.01... although you can choose which ones to install during the intallation process.
In addition you can also install the toolbar for developers to test errors easily.
Utilu IE Collection is an useful tool for webmasters and developers. If you want to know how people see your website when they visit it using different browsers, it's a good idea to check it personally.