PCS Webforms comparison to VBCS

Summary: In PCS you can create webforms that you can embed in a workflow. What are the differences with what you can build using PCS webforms versus VBCS?

For example .. if we would like to create request forms to add invoices, approve the request and after approval process the invoice in Oracle SaaS .. currently we build a VBCS form, use PCS to approve it, then call a webservice.

The request form contains a number of fields with value lists (fetched using get services) and a multi record block with lines.

Why shouldn't you simply build the entire form in PCS? Why use VBCS? Are there things you cannot do in PCS in the webforms?