Because if they were kept in wet places they will get convert into gypsum after that it will not come in use .
pop is used for projects and for repairing fractured bones when pop comes in contact with water it will turn into gypsum which is of no use so pop is kept in moisture proof containers 
hope it helps...........
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i am also from same class 10
i am also in search of this answers
if u get those answer plz tell me also