local -> local file copy question

[Linda Walsh]

Justin T Pryzby wrote:
> Note that --whole-file "is the default when both the source and
> destination are specified as local paths", and implies that the file
> is copied without the "delta transfer" algorithm.
---

I am using the 'whole-file' copy, 'explicitly', as I didn't know
that was the default in a local->local file copy.

If that algorithm was intended for local-local copy, how does
it use breaking up a file into blocks and summing them help
file transfer, over, say, spewing the entire file in 1 write
over a pipe like say cat a | cat >b would ?

Alternatively, if it 'knows' it is local-local and it is copying the
whole file, how does the block-summing copying of the data through a pipe
to another process  providing benefits, over say, simply writing the
buffer, directly back out to the new file location, like a 'cp' command
conceptually, might do?