The pager control was introduced with the common controls library that came with Internet Explorer 4.0 in order to assist in implementing scrolling menus on the Start menu and Favorites menu. (The Start menu and Favorites menu aren't really menus in the Win32 sense. They are custom controls written to act like menus; the fakemenu sample provides the basic idea.) The menu part is wrapped inside a pager control, and it is the pager that provides the scrolling behavior.
It so happens that scrolling menus were a bad idea. User feedback after the release of Internet Explorer 4.0 rather decisively indicated that people much preferred multi-column menus on the Start menu, so the default was changed back to multi-column menus in subsequent versions. The pager control is still used on the Favorites menu, and it was publically documented, so the control cannot ever die (for compatibility reasons), but not much attention is paid to it any more. It takes its place among the forgotten common controls.