From the GPL FAQ[1]:
The reason we require the offer to be valid for any third party is so
that people who receive the binaries indirectly in that way can order
the source code from you.
Reading the context, this is only the case if you choose to distribute
the binaries without source (but
I understand it *does* mean that you have to be the one sending the
source. If you distribute a GPL'd binary without source you have to
include a written offer to send the source to any party that receives
that binary, even indirectly. In general it should be easier to
include the source in
