The use of the feather function is most likely to make the edges of your selection smoother instead of hard. Try it out.
Not just 'smoother' but quite blurred, the more you feather, the more the edge of the selected area spreads out, I use it mostly to make background glows and shadows under objects (different from a drop shadow) for that 'web 2.0' feel.
Feather function is to blurs edges by building a transition boundary between the selection and its surrounding pixels. This blurring can cause some loss of detail at the edge of the selection.
when u use feather function its like smooth design .... not have any pixel ur output should be smooth.