To know what CD is used for, one should first of all consider what kind of molecular characteristics give rise to the CD signal, in order to be able to relate an experimental CD spectrum to some sort of useful information concerning the molecular system under study.
As for any other absorption phenomenon, the rise of CD is linked to the presence of particular chemical groups able to absorb the incident light of selected wavelength, i.e. a chromophore.
Moreover, this chromophore must have the special property to absorb differently the left- and right circular polarized light, i.e. it has to be non-symmetric in its absorption behaviour.
As one can imagine, chiral chromophores as well as chromophores linked to a chiral center behave exactly like this.
For an example of such a class of CD-active compounds, look to the citydine reported here.
However, there is another source of CD signal: the chiral disposition of chromophores in a 3D structure. To understand this, think of a helix. The helix has a handness, so its specular image cannot be superposed to it (by the way, having a handness is literally what chiral means in Ancient Greek).
If a group of chromophores is disposed in a helical arrangement, for example the way the amidic bonds in an alpha helix are, there is a pronounced CD effect.
Specular compounds have specular CD spectra: if one has a reference spectrum to compare with, the chirality of related compounds can be derived. This simple "organic chemistry" application was sufficient to achieve one of the triumphs of circular dichroism spectroscopy: the discovery of the Z-DNA form as a very different structure in comparison to the A and B forms. Well before the detailed structure of the Z-form was known, CD spectroscopists were able to predict correctly the handness of the helix (and to discover the Z-DNA as well).
However, CD spectroscopy is a technique suitable to study many other aspects of the molecular solution structure of biocompounds.
