to allow a layer of warm air and allow your thermal clothing to stay uncompressed and work. Without air in your drysuit, the water pressuer outside the suit will compress the suit against your skin and your body heat will move quickly to the outside of the material. You will be dry but very very cold. With air you can maintain your insulation.

Yarp, That too. :-)

@@TheCryptKeeper8 That and don't be squeezed to death.