JFace is defined by the Eclipse project as "a UI toolkit that provides helper classes for developing UI features that can be tedious to implement." It is a layer that sits on top of the raw widget system, and provides classes for handling common UI programming tasks. It brings model view controller programming to the Standard Widget Toolkit.
You can find more info at : Jface Wikipedia
The UI creation is simplified by using Jface....internally Jface uses SWT widgets only...but JFace reduces the complexity of dealing with complex UI creation by providing helper classes.
And Yes SWT does provide many functionalityfor creating as complex UI as possible.