why we use @Master type?

Question

if we have a master page and a content page.so the content page @Page directive look like as

<%@ Page Language="C#" MasterPageFile="~/Site1.Master" .... />

so , in order to access master page controls in content page we should have to use

<%@ MasterType VirtualPath="~/Site1.Master" %>

so , my question is this why we use @MasterType directive when we already define in the @page directive that this content page is in the master page (here -- Site1.Master)