Baths do wonders for stuck sheds. During the early winter months, my girl went through a few months of non-stop bad sheds, so I would bathe her before her shed also to help. Also, snakes can absorb a good deal of liquid through their vents, so a couple good soaks can help dehydrated snakes as well.
But, unless your snake has a stuck shed or will have a stuck shed soon, baths are absolutely not neccessary. A soak dish should be available in the enclosure, so they can drink and soak if they want to.
If you do decide to bathe your snake, the water should be warm to your wrist and about halfway up your snake's thickest point. Many people suggest putting them in an enclosed container with a lid and leaving them, but there have been multiple drowning incidents, so I don't reccommend it.