You need a humidity indicator card if you are concerned as to whether or not the desiccant in your package is still active. Desiccant becomes inactive when it has adsorbed as much moisture as it can for the conditions it is being exposed to. This can occur under the following conditions:
If your package is not properly sealed.
If you don’t use enough desiccant in your package to draw down the relative humidity.
If the barrier material (e.g. plastic bag) you use to package your product has a high moisture vapor transmission rate (which means that a significant amount of humid air is entering your package through the barrier material itself).