I haven't used VS2008 but my guess is that the purpose of this is so that you can place meta tags (or anything else you might typically put in the html header section) that are related to your content page.
Example:
<asp:Content ID="HeaderContent" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
<meta name="keywords" content="keyword1, keyword2, ..., keywordN" />
</asp:Content>
If a post helps me I'll always eventually mark it as an answer. But I frequently don't mark it right away because I feel once a thread is marked as answered, discussion tends to end. And I like to discuss things a bit.