The ziplock bag, when sealed, keeps the humidity level inside the bag about as high as it was when it was sealed.
Originally Posted by Steve Smith
Let's say you sealed it on a hot, humid summer day. Come wintertime when it is cooler the humidity is trapped in the bag but the dewpoint has fallen far enough the moisture will condense on the camera parts. Note that a lot of moisture can accumulate before you ever see it. Once you can see the condensation it is very severe.
You have also created the perfect environment for fungal growth.
Leaving the cameras to breath avoids this issue with seasonal changes.