"The "blind" 15mm end is designed to be used with an olive as a compression fitting in something like a radiator valve. If you want to join the 15mm end to a 15mm pipe you should use a different fitting. This is a cheaper way to use the less expensive 15mm radiator valves so that you can reduce the pipe work to 10mm."
"Why should I have to use a straight fitting to use this reducer? The 15mm end is 15mm outside diameter, which means having to use a straight connector to complete the joint, which of course adds to the cost."