It is the only way to present data in a consistent order.
It is not strange for some. For example, I can easily imagine that documents already in the server cache are sent to the client first while the server reads other documents from disk to prepare the next batch. Otherwise, you flush the current documents from the cache to read the documents of the consistent order, and then you reread the documents that were already in the cache because they are next in the consistent order.
If you do not sort, the server assume rightfully that you do not care about the order, and make sure it does the least work possible to handle your request so that it has more cpu cycle to handle requests where order is important and specified.