I use 'em for complex (large) selections, such as a fullscreen scanline selection, or grids… anything really, plus they give an easier way of using the displacement and lighting effects filters.
I use 'em all the time 
example (for making a square with rounded edges, using channels):
Make a square/rectangle on a new channel layer. Gauss blur the bastard with at least 5, I use 10. The higher the gauss, the softer the edges.
Then go image->adjust->levels, and play around 'till the edges go sharp again, then ctrl+click the channel, go to layers palette again, make a new layer, and fill yer selection with the color of choice.
That’s how I make soft squares USING CHANNELS, there’s a MUCH easier way of doing it, this is just an example.
