If it is mainly for your truck I would even consider leaving a couple of cans of soup or stew in there. If you have to leave your truck you do not have to take the cans with you, however if you do and they are empty (and you are now full
)they might come in handy as other survival tools.
The maid reason I like a purifier is that it usually does not run out. You can use it multiple times and they are usually meant for back packing so they are light and easy to carry. I have carried one for the past few years while hiking and hunting and I can tell you it is nice to only have to carry one thing of water and know that I can refile it at my convenience.