The head ContentPlaceHolder allows us to declaratively add custom markup to the master page's <head> region. Custom markup may also be added programmatically. Recall that the Page class's Header property returns the HtmlHead instance defined in the master page (the <head runat="server">).
Being able to programmatically add content to the <head> region is useful when the content to add is dynamic. Perhaps it's based on the user visiting the page; maybe it's being pulled from a database. Regardless of the reason, you can add content to the HtmlHead by adding controls to its Controls collection like so: