This is because of the variation of this pose when you sit upright hold the feet and gently bounce the knees up and down. The knees are then imagined as the wings. Cobbler Pose is another name for this pose, because it resembles the sitting posture of a cobbler at work.
Besides all that, it is a therapeutic hip and groin opener, it can boost your fertility because of the increased blood flow to the pelvic area.
It is important to have the pelvis in a neutral position, so sitting on stacked blankets etc might be necessary to make that happen. When the pelvis is able to tilt forward, you can come into the forward bend variation like in the picture. Otherwise, hold on to your feet and keep sitting straight up.
In the full pose, the knees are on the floor, but in reality that isn't happening for a lot of people. Accept that your knees can be anywhere from as high as your shoulders to half way down or lower! It's all ok, you will all still get the benefits.
Don't force the knees down in this pose, rather release the head of the thighbones (top of the thighbones) to the floor, and let the knees flow, following that action.
You can stay anywhere from 1-5 minutes in this pose.
Enjoy!! And practice for a few weeks to see how your flexibility increases in the hip joints..
Here is a step-by-step description of Butterfly Pose and more of the benefits listed.