WPD Vs PD Environment

Hi List,
Sorry for asking what may be seen as a Novice question but I was wondering what is the use of having a WPD and a PD environment together. Are there any advantages or disadvantages in using them both?
As far as my understanding goes the WPD environment is used to load balance. Where are the applications, business functions run in either case?
Please someone explain the advtg or disadvtg in either case.
Regards,
Ossidon

Veriy simple explanation: both environments have different OCMs. PD maximum loads (runs business functions and UBEs) client workstation and WPD maximum loads enterprise server.
Every BF can run on client only or server only or may both. OCM redirects running BF maximum as possible on server in WPD.