The reason to use baking soda in addition to acid is if you want to increase the sodium content without adding chloride. My water starts with a pH of 9.3-9.6, so I certainly don't need anything to push it higher, and often need acid to make it lower. Baking soda is not something I use very often, but my water is also low on sodium so there are times when I add some for that purpose.
--GF