When we talk to the emperor for the first time, he tells us:
I remember meeting with a bamboo seller called Kaguya. And I remember a secret treasure for crossing a sea of fire...Then, did they lock her up in order to get the Fire Tablet, so they could cross the lava pit around Himiko? But that brings two problems. 1) Ninetails could cross it later just fine. Then again, perhaps s/he couldn't do that without the foxrods, and they had her as a backup. Besides, s/he has flying minions, right? Or it was for Blight's use, perhaps. Let's just assume that was why they needed it.
But then we have 2) If they really wanted the Fire Tablet, why didn't they snatch it from her when they locked her up? Are they that stupid, or was it that Kaguya wasn't carrying it with her at that time? Didn't they even torture her for its whereabouts?
I need to check the dialogs at Sasa Sanctuary to reflect more about this theory, but I have no time to search for videos now. Oh well, maybe some other day...