In addition to increasing the depth of field in order to keep everything in focus, many lenses are also less sharp at wider apertures. Landscape on digital is best shot in the range of f/5.6 to f/11. Anything wider (faster, smaller number, bigger aperture) than f/5.6 will start to "soften" the image, and anything narrower than f/11 will lose definition to diffraction.
The main advantages to using a wider (faster/smaller number/bigger opening) aperture are:
- Let in more light to reduce the exposure time, to reduce the effects of camera shake, subject motion, and low light
- Limit the depth of field in order to have subject in focus and background out of focus (better subject isolation)
Disadvantages of using wide aperture:
- Details are less sharp
- Subject can be out of focus
The advantages of using a narrower (slower/bigger number/smaller opening) aperture are:
- Better lens sharpness
- Larger depth of field - capture both close and distant subjects in focus
Disadvantage to narrow apertures:
- Requires longer exposure time for equivalent exposure, so subject or camera movement can cause blur
- Poor subject isolation - the background being in focus might reduce artistic quality of image
As landscape is generally not moving, and often mounted on a tripod, having a fast shutter speed isn't important, so you can use a smaller aperture in order to get sharper images.
You can test this yourself - set your camera aperture priority, point it at a still subject and take two pictures - one at the widest aperture setting on your lens, and one at f/11. Blow them up on the computer and the difference will be apparent in details.