The reason for this is difficult to say, but in some cases it is to prevent the media and competitors from learning that a buyer, such as a large corporation, is considering purchasing real estate to relocate its headquarters or build a new factory.
The fact that the CA or NDA was submitted is itself a confidential matter, and the seller or the real estate brokerage company involved in the transaction must not leak it to others, including the contents of the confidential information contained in the CA or NDA. However, in reality, there is no guarantee that those confidential matters will be protected.
It is also assumed that the purpose is to prevent the CA or NDA submitted by a potential buyer from being used as one of the materials for negotiations with other potential buyers. For example, when a real estate brokerage company on the seller's side or the buyer's side receives the original CA, it is possible that they may make a copy of the original and use the copy in negotiations with other potential buyers.