Less weight and saves space. Note how in the post image it mentions they’re good for hiking, weight and space are a big thing when packing for hiking. They expand after use yeah, but you might not use all of them. So you have them just in case but still generally save space.
Is it really less weight though? If you have to carry in the towels and carry in the water, then combine them, isn’t that the same as carrying them already combined? Same thing with space. You’re compressing the towel but you still have to carry in the water.
You already have to take water anyways even if you take regular wet wipes, and you should already be taking some extra water just in case. You don’t need as much water for these towels as wet wipes use, and in a lot of cases you can use water from the environment (like a river/stream/lake/rain/etc) so long as you’re not ingesting it or putting it on wounds, say if you’re using it to keep cool or clean yourself. You can also use excess water from what you already use for other stuff like if you’re washing dishes or washing your hands and so on. So you don’t really have to take more water than you would have anyway. Some hiking trails provide occasional stations with potable water as well, at least where I live, particularly if they’re near/around a campground.
If there’s water available you don’t have to carry it in. Treatment tablets and filters are lighter than water, and let you make potable water as you need it.
