Primarily created for Firefox versions from 29 where there is no separate bookmarks sidebar button.
It becomes useful for me when it requires a relatively longer time to press the Ctrl+B instead of pressing the button with my mouse. E.g. when my left hand is busy or is resting or I just want to use only the mouse instead of typing every time something with the keyboard...
For me I position it as the first toolbar button so that it be closer to the sidebar. But on further updates it may jump to the last position (cause by default the button should have this place (according to this site rules)). So, if that happens, just move it to the place where you had it before.