Many digital photographers only reach for a wideangle lens when they've hit the limit on their standard zoom. One benefit of going wider is being able to squeeze more of the scene into the image. It's great for landscapes with big dramatic skies, and equally useful for interiors where you're physically constrained by the walls of a building.
But there's more. Wide-angle lenses are brilliant creative tools for exaggerating perspective. Move in really close to the main object in a scene and the middle distance shrinks away at an alarming rate. The results can be truly eye-popping images from a multitude of different scenarios. Portraits of people in their surroundings work particularly well.
If you're only using a wide-angle zoom lens at or near its shortest focal length, the versatility of having a zoom is largely lost, so there's no good reason for potentially degrading image quality by using a zoom instead of a prime. With the simplified design enabled by a fixed focal length, prime lenses typically deliver less noticeable distortion.
Ultimately, you're not gaining much in terms of outright viewing angle, if anything at all, by switching from a standard zoom to one of these wide-angle primes. However, along with a noticeable reduction in barrel distortion and other optical aberrations, there's a significant bonus in terms of speed.
Nearly all of the full-frame compatible prime lenses in this test group have a 'fast' widest available aperture of f/1.4, apart from the Nikon 28mm lens which still offers a respectable f/1.8 widest aperture. Aperture widths are still impressive for most of the lenses designed for crop sensor cameras, including the Fujifilm 18mm f/2, the Olympus 12mm f/2 and the Panasonic 14mm f/2.5. The odd one out is the Pentax 15mm lens with a widest aperture of f/4. That's actually three full f/stops slower than the f/1.4 lenses.
What's so important about a wider aperture anyway? The ability to reduce depth of field isn't normally a requirement for wide-angle lenses, although very wide apertures can still give the possibility of blurring the background when using these lenses at or near their shortest focus distances.
A more popular benefit of wider apertures in wide-angle shooting is to enable faster shutter speeds for freezing action and for avoiding camera-shake. It can make a big difference in dull lighting conditions, and for handheld shooting indoors or at twilight, enabling sufficiently fast shutter speeds without having to push the camera's sensitivity settings too far.
Ultimately, wide-angle prime lenses should give excellent image quality and are useful for an incredibly diverse range of indoor and outdoor shooting requirements.