Why was the butterfly needle designed?

The butterfly needle enables the proper position of the needle (parallel to the vein or artery with the bevel of the needle facing upward), it provides an expanded surface for taping the needle in place, and it allows for proper holding of needle while inserting and removing the needle.

In addition to Dr. Z's explanation, I might add that when I was a house officer we used rigid 20 gauge needles to draw blood and when confronted with infants, elderly patients with fragile,thin veins, need to access thin veins on the back of hands, the task was often impossible. How I would have loved to have been able to work with the butterfly needles!