One way to customize a PeopleSoft page is to execute javascript to manipulate the DOM. Some possible scenarios that may make this necessary:
- add dynamic elements to your page (set some elements as draggable maybe? or perhaps adding mouseover behavior to PeopleSoft page elements)
- removal/manipulation of elements not accessible via Application Designer (automatically added elements outside of a page definition: navigation elements, Close/Ok/Apply buttons in modal pages, etc.)