So why are we all buying bleach when the pool store chlorine is cheaper?
1. It's not available in all locales.
2. Many people don't have a pool store located conveniently close while almost everyone has easy access to a grocery store and visits one at least weekly.
3. Relatedly, you don't have to make another trip to another store to get bleach; you can just pick it up while you're at the grocery or Wal-Mart or wherever.
3. The 12.5% stuff, because most people won't use a whole container at once, whether it's stored in carboys or jugs, requires more handling with a greater potential for spills and splashes that will damage clothing and other things.
4. The stronger stuff degrades more quickly.
Store it in the garage or the basement of my store. I don't like stuff over 10% as it degrades very quickly. Further, they require that you give them a deposit on the containers and it is just a pain. When bleach is on sale at the market it is the cheapest form to buy.