Continuing professional development (CPD) or Continuing professional education (CPE) is the means by which people maintain their knowledge and skills related to their professional lives. Here are some of the professions which require continuing professional development.
Since accountants and auditors are not included in the list, here is some information on my understanding of the need for CPE/CPD:
For accountants, continuing professional development should be relevant to the practice which you are currently engaged in. Thus while you may undertake all types of professional education, application of the training adds value in that you use the training and develop professionally. I guess that is key to understanding the reason underlying the need for continuing professional development.
I am not sure if this requirement is to widen the scope of work which can be undertaken by the professional as there are different areas in which to develop while working to obtain membership in the association, such as tax, accounting and auditing. After obtaining membership, unless you are great with research, or willing to work extremely hard to develop knowledge outside of the area of expertise which was initially obtained, becoming functional in any of these areas takes a fair amount of work.