That "old" method (although still taught by the Pool Stores so they can sell the stuff) is kid of a band-aid just in case. For people that only use tablets, the CYA continues to rise to the point where they do not add enough FC to stay at adequate levels in proportion to the amount of CYA in the pool. Adding "magic powder" every wee gives a temporary boost in FC (although also adds either more CYA or CH) which may delay the pool turning green. Eventually, the pool store will say something like "you have chlorine lock" or "Your CYA is too high" and recommend draining and replacing water ... and then start the cycle over.
If you maintain adequate FC through accurate testing and keep the CYA in the recommended range, you will never have to go through the shock process and likely will never have to replace water in the pool.
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