why is it desirable not to completely empty a compressed gas

For compressed gas cylinder as a purified source of gas, it is desirable not to completely empty for two reasons :
1. If it is completely emptied up to tle level inside pressure is about the same as the atmospheric pressure, other gases (as impurities for the original gas) will flow back into the container and will cause problem when the container is refilled by the gas company. (Problem of purity)
2. If the gas is combustible, the danger is even larger. E.g. hydrogen, acetylene etc) If it is completely emptied up to tle level inside pressure is about the same as the atmospheric pressure, oxygen from atmosphere will flow back into the container and may explode. (Problem of explosion)

I hope this can help your understanding. =)

gas company dislike this