Cocoons are dried by using the process of stiffling. That is done in order to protect them from damage due to long durations of storage. The required conditions for reeling of the cocoons is better preserved by stifling.
Otherwise the metamorphosis continues in the pupa of cocoons and it will not be suitable for reeling. The stifling kills pupa and evaporates all the moisture.
This is part of sericulture. In tropical areas like India, steam stifling or Sun drying are used.