for my 2ยข worth.
- no container it "perfectly" safe. since water seems to be the universal solvent, chemicals from the container leach into the water. flushing it out from time to time keeps the chemical load to a minimum. I've had enough metallic tasting desalinated water in my life - the flavor was provided by stainless steel.
- no water is pure (outside of very carefully controlled conditions) - replacing it from time to time lessens the probability bacteria will multiply to an extent the water becomes dangerous to drink. -have some fun. put a jar of water in a sunny window sill and keep it topped off. see how long it takes to turn green
want some real fun? look at a sample under a microscope the bacterial load is awe inspiring. sealing the water just seals in what it came with.
- replacing the water from time to time is a good maintenance practice - you can check for leaks and vessel / seal integrity. - black mold growing in the wee corners of your basement or shed can be annoying.
there are probably a list of good reasons - but the above are the ones that came to mind first.
