Because you can write new 'plugins' to expand the abilities of Skiin. A Skiin's plugin is made of few functions that will be used to export / import the weights / deformers informations. If you feel comfortable with scripting you can also have a look by yourself in the 'Skiin_rsc/plugins' folder, there is a _model.py file with explanations
It allows you to store in external files (.skin files) all the datas you want to extract from your Maya's nodes. For instance, the most common usage is to extract your skinCluster weights, you can save the weights in an external file, and paste them on a new mesh, in a new scene, wherever you want, super easily.
It also allows you to do the same process with the objects which modify your object, such as ffd, cluster, whatever you want actually, even the constraints if you want to ! This can be a good way to export everything you did on a rig / skin process in one external file.