I highly recommend reading the book your body's many cries for water..it has a chapter on the many many reasons to take salt and why it is needed if taking large amounts of water...there are two systems of hydration one within the cells and one of the outside (Of the cell/s related to osmosis and when ewe do not take salt with the water (do not have to take it with the water just so much for every so many cups) then our emergency system for hydration will not work leading to many symptoms.
He says water, salt and potassium are the most important things for the body..this book will show you just how many things are related to not getting enough water (and salt) and how we have many more symptoms of us being extremely dehydrated such as pain before the dry mouth shows up