The particle group node takes care of storing any Effex particles that are generated inside of the framework.
Furthermore it defines the global behavior of the different particle types.

Overall there are four different predefined particle types (and therefore four subgroups in the particle group) with different purposes but they are not restricted to these purposes. It's only for user convenience that these particle types are defined but you can use them, like any other part of the framework, for any purpose you want.