I think buying soap online can be useful if you are buying soap for a business like a restaurant, or a school because most buildings and schools have a storage room which is full of supplies like soap, paper towels, and office supplies.
Saving a few hundred dollars a year by buying soap which is on sale on Amazon, eBay, etc can be worth it for businesses and schools which uses a lot of soap per year. You are also saving money on gas when buying soap online if you need to drive many miles to buy soap and cleaning supplies. The money saved on buying cleaning supplies online can be used for other more important things like hiring an extra worker to help out with work at a business, or buying sports equipment like soccer balls at a school.