OK, time for a pop quiz: The most obvious reason to own a really wide-angle lens is to take in really big spaces in one shot--right? Wrong! Well, only partly wrong actually. Yes, you can use a wide-angle lens to capture a wide, sweeping view like the Grand Canyon or a mountain range. The problem with shooting views like that with a very wide-angle lens, however, is that everything in the shot gets smaller and the view tends to lose grandeur rather than gain it.
In reality if you want to take in expansive landscapes or scenics you're better off shooting a series of images with a normal or moderate telephoto lens and stitching them into a panorama. Panoramas shot with a normal or slightly telephoto lens usually offer a much more realistic interpretation of a wide view or vista.
So what's the point of owning a super-wide-angle lens (a super-wide in 35mm terms would be anything wider than about 21mm and as wide as 10mm or so)? For me, the beauty of a super-wide is its ability to stretch and elongate objects or to really exaggerate the spaces between parts of a scene. If you photograph a row boat sitting on the beach using a normal lens, for example, the boat will look pretty much like it looks to you without a camera in front of your face--the proportions will seem very natural. But if you photograph that same boat (shooting from a very close perspective of, say, a few inches from the bow) using a super-wide lens, the boat will seem much longer than it really is and its exaggerated shape will dominate the design of the image.
I used exactly that idea photographing these giant saguaro cactus outside of Tucson, Arizona. Using a 10-20mm Sigma lens I knelt down and, from a vantage point just a foot or so from the base of the catci, I shot almost straight up at them. The already-tall saguaro now tower over their environment. It's not a method (or a lens) that I use all that often, but when I do I'm always jazzed by the interesting look it creates.
As I said, you can also use a very wide lens to exaggerate the spaces between near and far--which can be quite an effective tool in landscape photography. If you're photographing a farm scene with a tractor in the foreground and a dirt road leading to a barn behind it, you can move in close to the tractor and it will dominate the shot, making the road seem long and the barn seem much more distant. That type of spatial stretching provides a great feeling of presence in a landscape shot, making the viewer feel as if they're experiencing the spaces involved.
One other great use of a super-wide lens is shooting in tight quarters--hotel rooms, small museum rooms, etc. It's nice to have a lens in your kit that, even though it will distort the dimensions (and probably the shape) of tight spaces, at least lets you include everything. Often too the inherent distortion created by the an ultra-wide-angle lens adds to the intrigue of the shot.