Since digital cameras do so well selecting settings for you, why would you want to step in a make changes? Mainly because there will be many situations where an experienced photographer will do a better job selecting settings than the digital camera itself.You may want a special color effect obtained by adjusting the white balance. Or, maybe a black and white, sepia or any other color filter you camera has to offer.
You may want to isolate your subject with a selective focus,with fore and back ground objects blurred and a clear sharp subject.This can be acheived with a combination of a large F-stop, manual focus and a telephoto zoom setting.
Perhaps you would like a certain shutter speed to produce a special look. With a slow shutter speed you will produce a fluid blurry look in running water. If you want to freeze the action, you will want to use the fastest shutter speed available.
You can create silhouettes using your backlight scene mode and adjusting your EV settings. This will help by not allowing the background to wash out and produce a true silhouette.