Originally Posted by
Quincunx
Why not though? The groove's just there to make space for the thread, what would it do to the stitch physics if there was just empty air there? Not much. A little less resistance on the fabric on that side is all...
The way a regular machine makes a loop for the hook or shuttle to pass through is that the side with NO groove allows the thread to be "pinched" between the needle body and the fabric so that when the needle is lifted the thread is held long enough for the loop to form. If you put the needle in backwards, the thread will be safe in the groove and will slide up with the needle so no loop = no stitch.
This is why it is so important for the thread to be going in from the correct direction and the needle to be oriented properly.