So Akainu forbade Fujitora to visit any navy base, until he captures Luffy. Fujitora still went to Maryjoa, against those orders. This is important for two reasons.
1. If he hadn't come there, the Revos would have had to deal only with Ryokugyu, so if Fujitora was really secretly working against the WG, he could just have followed Akainu's orders. He could have helped the enemies of the WG without taking any risk. He didn't. Same goes for Ryokugyu, who could have tried to throw Fujitora out, if he was secretly working for the WG.
2. One reason why Ryokugyu didn't try to kick Fujitora out by force was that Fujitora would have hit back. They didn't want to destroy the place. So Fujitora can't fight with full strength in Maryjoa or even in a navy base. He would destroy everything.
Oda wrote this conflict because he wants Fujitora to show his full power. Fujitora is not allowed to go back to the HQ. This rule will still stand when Luffy prepares to attack the navy base. He will stay somewhere close to New Marineford, but not too close. When Luffy arrives to fight Akainu, Fujitora will be in the way. Zoro will have to stay behind to fight him while the other Strawhats advance to New Marineford. Fujitora vs Zoro will be isolated from the others. Fujitora will have the space to unleash his whole power without fear of harming bystanders.
1. If he hadn't come there, the Revos would have had to deal only with Ryokugyu, so if Fujitora was really secretly working against the WG, he could just have followed Akainu's orders. He could have helped the enemies of the WG without taking any risk. He didn't. Same goes for Ryokugyu, who could have tried to throw Fujitora out, if he was secretly working for the WG.
2. One reason why Ryokugyu didn't try to kick Fujitora out by force was that Fujitora would have hit back. They didn't want to destroy the place. So Fujitora can't fight with full strength in Maryjoa or even in a navy base. He would destroy everything.
Oda wrote this conflict because he wants Fujitora to show his full power. Fujitora is not allowed to go back to the HQ. This rule will still stand when Luffy prepares to attack the navy base. He will stay somewhere close to New Marineford, but not too close. When Luffy arrives to fight Akainu, Fujitora will be in the way. Zoro will have to stay behind to fight him while the other Strawhats advance to New Marineford. Fujitora vs Zoro will be isolated from the others. Fujitora will have the space to unleash his whole power without fear of harming bystanders.