Content place holders in a master page can be useful things used for rendering controls on a page layout in a different area of the master page or to remove a control from a specific layout. Also it helps to get around not being able to add a web part or web part zone to a master page and then not being able to edit it in the browser ( even though it involves having to have custom page layouts if you want it to render).
The great thing is you’re not fixed with the Out of the box SharePoint place holders you can create as many as you want as long as the ID doesn’t conflict.
on the master page add the content place holder <asp:ContentPlaceHolder id=“CustomName” runat=”server”></asp:ContentPlaceHolder> and add this to your page layout <asp:Content runat=”server” ContentPlaceHolderID=”CustomName”></asp:Content>. You’ll now be able to place a control or web part into the page layout and it will render in the master page, So now editing the page you’re using in the browser will allow you to edit the web part or alow you to add one if you’re using it as a web part zone.