It also possible to say that Mask must be bought with the respirator, as Gas mask filters would probably have same rating as respirator, so it effectively nulls the price advantage of respirators with 250Y for the additional protection to contact etc toxins
Gas Masks specifically state they can't be used with respirators.
As for the reasons to use respirators, the 1 hour air supply is actually quite limited - if you're expecting a chemical attack, you usually don't know exactly when it'll be incoming, so to be protected you'd have to have the mask running non-stop. And if you're in a toxic enviroment an hour is always over quicker then you'd want it to be. Meanwhile, it only takes gas masks 3 refills to become more expensive then the R6 respirator.
Of course, the immunity may be worth all that - the extra cost isn't
that high - which is good, it means it's an interesting choice.