Sodium hydroxide, aka lye, is one of the most important basic chemicals used in industry, and until not too long ago, for several home uses. It can still be found in a few consumer products, but because of its usefulness as a chemical reagent for clandestine preparation of methamphetamine, is hard to get now without a legitimate business reason. However, I found some at Lowe’s a couple of years ago (under a different label) for opening drains. I wanted some to show my relatives how to make soap.
A significant amount of this material is still used for making soap, but its uses are so widespread and pervasive that soapmaking is just a small fraction of the applications for this material. Industrially it is used when a strong, cheap base is needed, because it is amongst the strongest and cheapest, other than perhaps calcium hydroxide (lime), but sodium hydroxide is very soluble in water where calcium hydroxide is not.