During the Battle of Scarif, why did Bodhi Rook and the rest of the company on the ground desperately try to contact the Alliance Fleet, just to tell them something they already knew? Surely the Alliance Fleet above the shield gate already knew they had to destroy it as that was going be the only way to receive the data. If they weren't trying to destroy the shield gate from the start, what else were they doing up there?
This is addressed in the scene where Cassian Andor speaks to the pilot Bodhi.
Cassian: Listen to me. The Rebel fleet is out there. You've got to tell them to blow a hole in the shield-gate so we can transmit the plans
Since the plans are too high density to be transmitted over normal audio channels, the shield needs to be down and the Rebels need to have a high gain receiver pointed toward the planet in order to receive the plans via radio since their chances of getting off the planet at this point are essentially zero.
K-2SO: We could transmit the plans to the Rebel Fleet. We'd have to get a signal out to tell them it's coming
If the fleet simply brings down the shield (expecting the ground forces to escape) that won't do any good.
K-2SO: There's fighting on the beach. They've locked down the base.
Bodhi didn't know they were there to destroy the shield gate, and that wasn't the only thing they needed to know. He was telling them they needed to destroy the shield gate in order to receive the transmission regarding the plans. They also needed to be ready to receive the plans since it would be a large transfer and will be on a specific frequency.
There may have been parts of the conversation we didn't hear, and there may have been a part where the Admiral tells Bodhi what specific frequency to broadcast the transmission so that his ship gets the plans.
Surely the Alliance Fleet above the shield gate already knew they had to destroy it as that was going be the only way to receive the data.
Maybe they did, maybe they didn't. Some communication was obviously possible through the closed shield, though the Death Star data was too much to transmit.
Jyn: We could transmit the plans to the rebel fleet. We'd have to get a signal out to tell them it's coming.
Cassian: It's the size of the data files. That's the problem. They'll never get through. Someone has to take that shield gate down.
There was no prior coordination of plans, and the Rebel fleet was getting hit hard. They'd tried to destroy the gate already. They had no idea what the ground situation was, or even if anyone was still alive. They could have chosen any number of possible strategies: engage the enemy fleet harder, cut their losses and retreat, try to delay the battle, etc.
Thanks to the communication with Rogue One, they knew that victory would follow seconds after the shield gate was destroyed. They focused anew on this objective, prepared to received the transmission, and narrowly escape d with the Death Star designs.