How bad is wet filament really?

Can you leave your PLA spools out in the open and do you really need to dry PETG before every use?
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Products shown:
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Prusament http://go.toms3d.org/Prusament
Eibos filament dryer:
on Amazon http://go.toms3d.org/EiDry
on Aliexpress http://go.toms3d.org/EiDryAli

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