At my friend's shop he has a Zhonghua cake with a plastic wrapper which just reaches around the edges. The paper wrapper is folded into a round shape and is inside the plastic. when you turn the cake over the leaves are exposed.
Perhaps in this way, the cake and the wrapper can both be examined without having to unwrap and wrap the cake back up. I personally don't see this as more convenient, unless you have a lot of clumsy customers who have a habit of dumping the cake or loose leaves on the floor when unwrapping display cakes.
A cake with a plastic cover with holes in it must be to protect form handling, as others have mentioned.
I fail to see the purpose of protecting a display cake in this way. Often display cakes are broken up to taste in the shop, also a display cake is hopefully not sold as ageing conditions are not ideal. Sunlight, too much air, etc will not allow the cake to age as well as those put away properly.
The only reason for doing this I can think of is the plastic wrapping is much quicker than the paper. Also it would be much cheaper, often pu'er paper seems somewhat high quality.