As discussed elsewhere, there are two types of lifecasts- Bucket Molds and Layup Molds.
In a layup mold, the alginate is spread fairly thinly onto the surface of the skin, like in a face or torso mold. As such the alginate requires a structural support so it doesn't distort when it is removed from the body. Putting on a hard shell of plaster bandages keeps the alginate in its original shape so you casting comes out better.
Plaster bandages don't normally stick to alginate so several strategies are used to keep them together. More on getting plaster bandages to stick.