They are different approaches. Part Design WB enforces the single part paradigm, ensuring that the part is always a complete manifold surface whereas Part WB allows a part to be assembled from discreet primatives. In PD merges are done automatically whilst in Part WB you must carry out any merges or joins manually.

Part Design can be helpful for beginners because it demands a particular way of working that always ensures a printable model but I found it frustrating being bound to that way of working and I prefer to use Part WB as my main process.