Before the actual launch, the planes need to be prepared.
To increase the size of an attack wave, which can be critical, the area after the 3rd catapult is important. It can accommodate at least three to four planes on Type 003.
One of the changes Shandong had over Liaoning is a new weapon elevator in that area such that the weapons don't have to be moved across the whole flight deck to get there.
Moving weapons is much easier compared to moving planes across the flight deck.
Then why not add an elevator for planes there?
In battle time, the deck operations are very complicated and almost a big mess. Launching, recovering, preparing, planes, weapons, etc etc, all at the same time. The efficiency is critical. And a 3rd elevator will definitely be a great help.