Whats the use of @ Register directives ?

@ Register Associates aliases with namespaces and class names for concise notation in custom server control syntax.
<%@ Register tagprefix="tagprefix" Namespace="namespace" Assembly="assembly" %><%@ Register tagprefix="tagprefix" Tagname="tagname" Src="pathname" %>
Attributes tagprefix An alias to associate with a namespace.
tagname An alias to associate with a class.
Namespace The namespace to associate with tagprefix.
Src The location (relative or absolute) of the declarative User Control file to associate with the tagprefix:tagname pair.
Assembly The assembly in which the namespace that you are associating with tagprefix resides.
Note The assembly name does not include a file name extension.