The Empty Capsule is a drug form (or galenic form) used when the drug (or any other substance to be used by oral way) it contains, has a strong odor or an unpleasant taste that you wishes to mask. It's also used when the drug has a texture not easy to put in the form of tablet.
Normally, the active product must be a dry powder, because the gelatin disaggregates when in contact with water. It's possible to putliquids in a capsule, but only is included in an absorbing substate (aerosil for example), or right before swallowing the capsule (it is the case for essential oils). Therefore, the Gelatin Capsules or Empty Gelatin Capsules are widely used. Nevertheless, the capsules can be filled with oily liquids, but in industrial environment only. It's possible to hide in a capsule a smaller product as a compressed tablet or another smaller capsule, for example for clinical trials (known as in "double blind").
Especially treated capsules "gastro-resistants" or with "enteric coating" allow to avoid the anactivation of a substance by the stomach acidity, or allow to obtain a delayed release, in the intestine.
The capsule is mostly a packaging form easy to realize, with a cheap material, from where its success in hospital, in pharmacies, and even at private individuals doing themselves their mixtures of plants. Thanks to the many combinations of colors that it allows, and with the possibility to print logo and reference n°, the capsule makes it possible to personalize a drug or a food complement, with a marketing aim for example. This personalization is also useful for the poisons centers to recognize a product when there is an intoxication There is also many other more "exotic" uses of the capsules. For example, they are used to store electronics components, or fossils of small size.