The explanation is that it WASN'T a closed system before it was gassed-up.
The cooling pipework, and the actual gas itself initially contains some water vapour.
The drying capsule removes this initial trapped moisture and it remains locked into the dessicant until such time as the sealed system is ruptured, either by a leak or by re-gassing.
At that point, during the resealing process, a new drying caspsule will be installed. Any water in the system would obviously corrode stuff from the inside, but would also no doubt cause problems to the coolant flow, by FREEZING and blocking the system.
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