I think he does it so the rest of the Akatsuki can be informed on what's happening. I mean, in a fight between akatsuki and some other ninjas, either the other ninjas will die and the akatsuki
member will report back to Madara, or the akatsuki member will die. In which case the rest of the group would be left in the dark. That is why zetsu watches the battles.
And the reason why he records the battles he watches is to learn about the enemy, maybe he will see a new, formidable jutsu, or maybe he will find out more about the capabilities of a countries military force. Either way, knowledge is power and knowing your opponents weaknesses is always good. I think the main reason zetsu records fights is so kishi wont have to explain how madara knows all about everyones abilities since hes never there. Like how he knew about kakashis thing before he even used it.
I think they need Kabuto's help is because Zetsu can only make a clone that has only a certain amount of the original's strength, Kabuto can bring a person back with perfect health and at full strength. They also can't be destroyed by conventional methods which makes Kabuto's ability superior to Zetsu's.