Little packets of silica gel are found in all sorts of products because silica gel is a desiccant — it adsorbs and holds water vapor. In leather products and foods like pepperoni, the lack of moisture can limit the growth of mold and reduce spoilage. In electronics, it prevents condensation, which might damage the electronics. If a bottle of vitamins contained any moisture vapor and was cooled rapidly, the condensing moisture would ruin the pills. You will find little silica gel packets in anything that would be affected by excess moisture or condensation.
Silica gel is nearly harmless, which is why you find it in food products. Silica, or silicon dioxide (SiO2), is the same material found in quartz. The gel form contains millions of tiny pores that can absorb and hold moisture. Silica gel is essentially porous sand.
Silica gel can adsorb about 40 percent of its weight in moisture and can take the relative humidity in a closed container down to about 40 percent. Once saturated, you can drive the moisture off and reuse silica gel by heating it above 300 degrees F (150 C).