Pagers are tiny programs that display a miniature view of all your workspaces and windows open on them. Usually they sit in the corner of the screen or in the slit.
A pager is different from iconbar or panel in the way that it does not show minimized windows and usually you cannot switch the active window. With some pagers you can drag windows from workspace to another (like from window menu -> send to).
If you have multiple workspaces, like I have 9, you can immediately see what windows are on which workspaces, and access them fast. Of course desktop wheeling is fast, but you may miss the workspace you want. Also a nice pager is good and useful eye candy ;)