dd: Purpose of the nonblock symbol for iflag and oflag

[Léo El Amri]

Hello,

I'd like to know what is the purpose of the nonblock symbols of the 
iflag and oflag options.

From what I understand after reading the source-code, it merely sets 
the O_NONBLOCK flag on the file descriptor, but does not _actually_ 
checks for the EGAIN errno.

What was the idea/use for this flag when it was introduced in 2004 ?

The man page for open(2) says that this is a noop for regular files and 
block devices. What error specific to using the nonblock flag would 
happen in dd if used on targets supporting the O_NONBLOCK flag ?

Cordially,
Léo