I couldn't possibly explain it better than wikipedia does: http://en.wikipedia.org/wiki/MIME_type
In addition to e-mail applications, Web browsers also support various MIME types. This enables the browser to display or output files that are not in HTML format.
IOW, it helps the browser (or content consumer, because it may not just be a browser) determine what content they are about to consume; this means a browser may be able to make a decision on the correct plugin to use to display content, or a media player may be able to load up the correct codec or plugin.