But, why can dam produce creature? (because I think that this is weird.)
Think of something like ...fishing?
Dam provides a place where Water creatures gather and live, ready to be brought on the field at your disposal. A greater reservoir enables larger Water creatures to live.
A more natural effect can be given instead, though I cannot imagine any.
And, why can dam produce aether quanta?? (another permanent that produce quanta beside pillars??)
It is thematically natural to think that a Hydroelectric Dam generates
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(electricity) and is an enhanced form of a Dam.
Also it is useful, since you can fuel Psionic Wave of an Ulitharid generated from the Hydroelectric Dam.