The Nu-Mantia intercept claims Towers were made as a "reversal of the congealing spiritual bleed caused by the Convention". The usage of the word "bleed" implies a gradual loss to me. My theory is that Mundus requires a constant influx of Creatia to exist: without Towers to supply it with Creatia, the world will end. The Intercept states that the first two Towers "allowed the Mundus to exist without the full presence of the divine". I assume that before the existence of the Towers, the gods had to channel all their power into maintaining Mundus. The "spiritual bleed" is caused by the Convention because that is when the Aedra stopped doing so, except for Lorkhan, who gave himself up entirely so his Heart could become the Heart of the World. Perhaps Auri-El's ascension was him reclaiming the full use of whatever divine power he had left, no longer needing to spend it on maintaining Mundus. The first two Towers were made to prevent Mundus from running out of Creatia without fully draining the gods (or at least slowing the time it takes for it to run out). I assume the "reversal" is that the presence of additional towers results in a net gain of Creatia, the opposite of a drain.

I also note that the Nu-Mantia intercept claims "the Altmer sought to focus on dracochrysalis, or keeping elder magic bound before it could change into something lesser (and act which ironically required aetherial surplus)". Presumably, Crystal Tower uses a portion of its Creatia on preserving the rest from the drain, acting as some kind of long-term Creatia storage.