The ones in food are often oxygen absorbers instead of dessicants. They contain iron "sand" that is, unfortunately, not reusable. They're usually very flat and have a "do not microwave" warning on them in addition to "do not eat".
(This is not to say dessicant packets aren't used in food, just that not all of those packets are dessicants)
(This is not to say dessicant packets aren't used in food, just that not all of those packets are dessicants)