The Egyptians compared the first rain, thanks to which the seeds of plants began to germinate, with learning, preparing a person for intellectual life: rain or dew served as symbols of the clarification of religious truths.
Maybe balance the future "getting wet" debuff giving some kind of buff? Like picking nettles rn, you take damage but also get a buff.
It would be possible to give a small buff to intelligence and will for a long stay in the rain.
Collecting rainwater can be justified as one of the quests for questgivers, for example. "Bring rain water." Or as a touch on the hypothetical credo of the Alchemist, and which, in addition to the tasks "collect x rainwater", there can be tasks "get wet in the rain" or "dry after the rain", here you can think of a lot of other things, of course. But the Alchemist's credo would be interesting idea.