We have medium to medium high humidity (average year round in the 60-70%), although not what @Renn_aissance experiences – it’s definitely not desert dry here. Most times I use the sheet, the benefit seems to come in the first 15 minutes; after that, if still needed, I re-wet it.
It takes less overall time to hose my horse off, if I count waiting until the sheet is dryish, once I get past the waterproof qualities of his coat (similar to a Labrador Retriever, lol), but with this sheet, I can wet it, put it on him, and leave him be, eating hay or whatever, while I do barn chores.
I’ve even been able to remove all of the belly and leg straps, using only the chest straps to keep it on (this is a very cooperative horse who seems to enjoy his sheet). I can dunk the sheet, squeeze out some of the excess water, and apply it to the horse, all in a couple of minutes. Without getting myself wet.
I consider this sheet as another tool in my toolbox.
eta: I wanted to add that, although our humidity level isn’t usually on the low side, we can generally count on a breeze. I think that makes a difference with this sheet.