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>