When you put food in a Mylar bag or vacuum package it or 'can' it in a canning jar [I'm talking dry food, not jam or pickles], there are two methods of ensuring that moisture and/or oxygen don't impact the quality and lifespan of the foodstuff: desiccants and oxygen absorbers (hereinafter, O2 absorbers).
There's a good reason to use one or the other; and you do need to separate them in a package, but do you know what they are for?
Practically everyone has seen the little packets that say 'do not eat' in something like a container of Parmesan cheese or even in electronics packages like the box a stereo component comes in; they've been around for years. These are desiccants, and their purpose is to keep moisture from damaging the product. Parmesan cheese, for example, clumps in damp air, and the desiccant helps prevent that.
Oxygen absorbers, on the other hand, are fairly new, and used to pull out the oxygen, which can impact the quality of stored food, from the air in the container. Sometimes there's confusion as to whether or not you need to use them and if you need to use a desiccant as well. The answer is no for some items and yes for others. If you do use both, the desiccant would be placed at the bottom of the container, then the foodstuff placed inside, then the oxygen absorber added. This is because the desiccant can actually impact the oxygen absorber’s ability to absorb.
Bugs in foodstuffs can breath oxygen, and oxygen promotes rancidity in some foods. Whole grains don't need O2 absorbers as much as processed foodstuffs do, but if you have a container of, say, whole wheat berries, you still have to deal with the dead space between grains as the whole wheat berries can't pack as densely as would salt or sugar. That's where the O2 absorber comes in; it will do the trick for that last little bit of residual oxygen. Here's a handy chart for determining how many O2 absorbers to use; note that the '100cc' or '500cc' refers to the size of the O2 absorber - bigger the number, the fewer you have to use.
Think of it this way: you have a jar or bag of something you want to store, and you live in damp Western Washington, and it's a rainy fall day with relatively high humidity. To keep the stuff from becoming rancid or buggy, you want the container's residual air to be thin like that at high altitude, and you want it dry. The O2 absorber will handle the air, and the desiccant will handle the moisture.
Note that when you use O2 absorbers, you need to remember two things: one, they have a finite lifespan and should be used when they are new, and two, if you only use a few at a time, keep your vacuum sealer out and handy and vacuum package the remainder to keep them usable. Desiccants can usually be revived simply by warming them according to manufacturer's instructions.