Well, one reason is that you can store CO2 in a tank for a long time, in lots of conditions, easily and with basically no cost. So, if either A) no stores conve

Well, one reason is that you can store CO2 in a tank for a long time, in lots of conditions, easily and with basically no cost. So, if either A) no stores convenient to you sell it or you need it at more flexible times etc, or for some other reason you want to it on demand at a price premium, it could be worthwhile. But, based on the write-up here, it sounds like he was doing this out of curiosity, at which point yes buying the CO2 would sort of defeat the point. Of course, one could also ask the same questions about buying the baking soda and acid. There is no clear delineation at which point before which it's "made from scratch" vs. "store bought". Sounds like the answer in this case was: the person in question thought that starting from purchased baking soda and acid was an interesting problem to solve, and starting from bottled CO2 was a boring problem to solve and starting from some precursor to baking soda and acid was too much of a pain to solve.