Silica, How does it work in a waterproof pack?

I am very curious, please answer if you know, my question is about SILICA, it comes packaged with most electronics, and countless other things and has the notorious label "do not eat" why it says that is anyones guess, but my question is why are the pebbles packed in a sealed plastic package? I am asking because I understood silica to be packed with these things to make sure no moisture contaminates the contents, but the silica is actually packed in a waterproof plastic package? what good is it doing? how is it supposed to absord moisture if its contained in a sealed pack??? inside the packaging of said object.