So Why Would The Planners Want to Have Extra Explosives on the Plane that Crashed into the Pentagon?

Two possible reasons:
One is that the plane that was used for the attack was a passengerless drone, and the planners knew that a normal drone plane would not definitely breach the reinforced walls of the Pentagon. If the plane didn't breach the wall, the plane would be clearly exposed as a passengerless drone when it exploded. Thus, explosives or a missile were planted in the plane to make sure the Pentagon wall was opened up upon impact, thus burying the plane wreckage in the building debris.

A second possible reason is that perhaps the plane was meant to take out some particular office or region or certain people in the Pentagon. In fact, the Office of Naval Intelligence was taken out by the attack, and this office was implicated in 9/11 by Delmart Vreeland. Therefore the plane attack on the Pentagon could have been a type of cover-up. The planners may have wanted to gurantee they could penetrate deep enough into the Pentagon to destroy the area they wanted gone, and thus put extra explosives into the plane to make sure it would cause the maximal damage. This would explain why the plane hit the relatively unoccupied and recently renovated area-- because it was targeting a special part of the Pentagon.