I have been wondering about this too… Suppose that you have a 10GB packaged game size, and after testing it is found that 1 blueprint is buggy and has been fixed. Does we need to repackage the whole thing? (time consuming, and sound ridiculous). It is nice to have the ability to just update 1-2 items in the package. I think in UE4 frontend, it has the ability to do it selectively but only on a map (not one single asset) - please correct if I am wrong.
I think the files have been scrambled somehow - but I am not sure.
All files are packaged regardless of them being used anywhere in the project. You must use ‘asset migrate’ trick to remove unused asset.