Dry fasting is 3 times more efficient than water fasting. The body uses it's fat stores for water as well as fuel and regulates our water needs as well as our nutrient needs. I found the main way I noticed this while fasting was that when dry fasting I would use my bowels regularly each day until they emptied, but while water fasting I usually stopped by the second day and didn't go again until the fast was broken. I believe that the body conserves what water we give it while water fasting but is unable to make up for not being given the right amount, just like if we eat a little bit we can starve but if we eat nothing at all- we are fasting.
And you do lose more weight also as a rule while dry fasting. It's only common sense, you must break down more fat cells for water. The benefits are just a faster more efficient detox. It can also be more difficult so that needs to be taken into account as well.
I have fasted for up to 7 days without any signs of dehydration. On the contrary all my dry fasts have been great experiences and improved my health considerably.