sweating vegetables

Following on from a thread about why you sweat vegetables, it came to my attention that people don't seem to know what the difference is, or in fact what either sautéing or sweating are. So I have found an article that explains the two processes from a culinary point of view. Good so far...

Any idea why we have to sweat some vegetables like onions and carrots? is it necessary or can we skip that process? I just follow procedures on the cookbook but did not find the reason for sweating and the use of salt for that matter. Some salt their fish before cooking I wonder if that has...