This function is very similar to the Move Face function. To make the difference clearer, use the Pull Face function to select the front face of the YZ plane and drag it as far as you like in positive x direction. (See figure "Selected Faces")

You will notice that the contour of the face does not change.

 

Now click on Cancel to return the component to its original state.

Now use the Move Face function to select the same face and move it in the positive x direction. The face no longer retains its original contour, but is continued according to the parameters of the features (in this case, the cavity).

The Pull Face function also allows you to move the enclosed face of a sketch. Furthermore, with the Pull Face function it is also possible to drag a face along all 3 coordinate axes, while with the Move Face function you can additionally rotate it by one point.