Tossing relatively small explosives against people (probably ) in trenches seems a really stupid wager when weighed against a very expensive pilot and aircraft.
The other reason to do so is firing for effect - you’re not aiming to hit anything (lofting unguided rockets is wildly inaccurate) but to force the enemy to stay in his shelters and remain static.
That may because your troops are either retreating or advancing in front of them or, with conscripts with low morale, to discourage them from following orders to advance as unprotected infantry across open land - as with the their last month of unsuccessful attacks to the south.