Much of North Africa is dry while hot during the summer months, and so are large parts of the Middle East. However, that applies just during the height of the day and along all the coasts as well as often quite far inland humidity levels rise relentlessly as the sun moves lower in the sky. In the late afternoon and well into the evening it can get so humid that eventually overnight you find condensation appearing on metal and many other surfaces. Usually the temperatures are still 25 degrees or more even when at their lowest and the combination of the two is quite debilitating. When the humidity is high at such temperatures you sweat profusely, making everything you are wearing damp and even dripping.
Of course you still sweat whenever it is hot, even if dry, but where the humidity is low it mostly evaporates directly, leaving your skin feeling gritty with the salts left behind. Dehydration can set in either way but that was not a problem I encountered and I generally drank less than the hyped formula - only when I felt thirsty. If any of it has affected my health then I can only conclude it must have made it better.
Of the places I have been to, inland (northern half) of Iran has the driest atmosphere, partly due to altitude, and there you do not notice sweating as much as in Saudi, Emirates, Libya, Sudan, Rajasthan etc. (although Saudi and the Emirates were the worst). All of these have their cold season (including freezing/snow) when sweating is not on the menu and heavier clothing is what you want.