You can also use it as a sneaky way to set the cookie on someone's machine. For example, if you wanted to redirect them to another site, but not to the actual landing page set forth by the affiliate, use a pixel.
Here's an example:
Code:
<iframe border=0 frameborder=0 framespacing=0 height=1 width=0 marginheight=0 marginwidth=0 noResize scrolling=no src="http://your-affiliate-link.tld/?sub=annie" vspale=0></iframe>
<script type="text/javascript">
<!--
setTimeout("window.location = 'http://www.true.com/profile/view_profile.htm?uid=16200972&p=b&idx=29&score=-1&linkid=29&Search_Guid=f58a58b4-80b1-4c0f-9dce-7e58612513cf'", 3000);
//-->
</script>
What this does is set the cookie for the True.com offer, but allows you to redirect someone to your real *wink wink* profile.