Why do the emergency oxygen masks for some 787 premium class seats have an extra 'streamer' to pull them down?
So in premium there's an extra step


For what I see in the picture you posted, not really.

There is always the step "grab the mask and pull firmly" to bring it from the compartment to your mouth (and break the supporting cable, that is now unneeded, in the process).

What I understand from the third picture (first row) inside the image you posted is that they made this step "easier" for the premium class, they attached a string/made the usual one longer, so that you don't have to reach up to the mask pulling yourself up, but you can make the mask come down the full distance by pulling the extra string that will come down near to you. Similar to what is shown in this other card found by mins.




  I can't see how this is not an extra step though. Economy = grab the mask, Premium = pull the string, grab the mask.


I see where you are coming from, but in the economy version you still have to pull.
I agree that having to grab two objects in a sequence rather than only one seems to be counter-intuitive for an emergency situation, but without seeing the mechanism in action we cannot say much more than this.
The leaflets seem to indicate that the "premium" solution is made to be more comfortable to reach, nothing else.