There is considerable variance in the use of the tabs. Most Griswold "economy" lines, Iron Mountain, Merit, etc, do not have the tabs. Early (Erie, Slant Erie) covers do not have tabs. Later Griswold-marked covers (typically unmarked) do not have tabs. Even some lettered covers don't have tabs. Other brands do not have them.
Seems like whatever the rationale was, their presence was not that significant.