what is the advantages of adopting content digest in docker?

Question

1. With introduction of CAS(Content Addressable Storage) in Docker 1.10,

   1.1 why and how data retrieval is improved

   1.2 In what context does security improve

   (It is stated that the physical location of data and its copies are hidden from users.

   Can someone please explain this in detail. Doesn't CAS(Content Addressable Storage) also use a mapping table that maps hash index to physical location of stored data? I don't understand how security is improved)

   1.3 What is the advantage of using content digest.

   (Since same hash is returned if contents are same, it prevents from downloading duplicate layer in Docker Hub where global repository ID is shared. However, not sure what other advantages are there. )

2. I do not understand the following statement "CAS uses concept of searching for data based on content instead of storing and searching for data based on directory" Is there any other ways to searching data other than using directory?

3. It is stated that "Docker Image ID collision is avoided using built-in method". What is this built-in method in detail? ( Is it common open-addressing or close-addressing??)