Preemies are so under developed that the slightest variation in temperature, sound or light can stress their already stress bodies. Every little thing that can add one more day to their lives is one more day to get stronger, and another day to grow outside the womb.
An incubator cover reduces the stress of the bright hospital lights to the baby that the doctors and nurses need to do their job. The darker the backing fabric is, the better the light reduction. It also muffles the sound in the room for their delicate little ears and may mean a little more uninterrupted sleep to grow and heal. Heat rises and the babies have trouble regulating their body heat, so the cover can act as insulation to help keep more heat in and slow down heat loss much like the little hats do, but for the entire body.
This is another projet that can be done with 8″ squares that are knitted or crocheted like the Pocket Scarf. Here is where you can print the Check with your local hospital to find out how they may want you to vary the directions, such as larger or smaller or different placement of loops for securing.