What is the purpose of having a wide angle lens like the Nikon AF-S 14-24mm f/2.8G ED with a low f/stop? For example I am shooting a landscape mountain scenery shots or group photos, why would I use f/2.8 when instead f/8 would be better so that depth of field will be large and every part will be in focus.
The reason I can think for f/2.8 in wide angle will be shooting in low light.
Is it reasonable to shoot landscape mountain scenery shot/group photos with f/2.8?
The most common use case for large apertures on a wide angle lens is in low light, especially wide angle views of the night sky. Because of the Earth's rotation and the apparent motion of the sky, shutter speeds need to be limited to about 600 seconds/(focal length in millimeters x crop factor). Higher ISO settings and the resultant lower exposure values tend to generate noise that is just as bright as many dimmer stars, and thus any applied noise reduction can also "eat" stars. Since depth of field is not an issue with celestial objects that are at infinity focus, wider apertures allow lower ISO settings than would be the case with typical apertures used for landscape scenes.
In general, even when shooting landscape scenes in low light the preferred method is to use a tripod that allows for longer exposures of static scenes. Even if there are elements moving in and out of the scene using narrower apertures and the resulting longer shutter speeds allow any unwanted elements that move fairly quickly through the scene to not show up in a long exposure.
When shooting large groups with wide angle lenses the narrower aperture is also usually desired to insure that everyone in the photo from front to back is in focus. If the group is fairly equidistant from the camera and the background is "busy" a narrower aperture can be used to separate the background from the subjects but care must be taken that focus is accurate and the depth of field is centered on the group.
Well usually for landscape or (group) portrait you want everything to be sharp hence at least f/8, so you don't care about f/2.8 or better. Or do you ? For example, imagine you see a nice flower in a field, if you take its picture close enough with f/2.8 or lower, it will stand out more against its background (grass, trees, forest, mountains, whatever) because the background will be a nice blur. At f/8 the background will not be (as much) blurred. Same to detach your portrait from its surroundings.
This is true for any angle including wide, but it becomes "truer" (pardon my French ? ;-) as the focal length increases.
Also the smaller f/2.8 allows you to take a picture at an "appropriate" (depending on the subject...) shutter speed without raising the ISO (thus the noise), or taking handheld at dusk or dawn without a tripod (if you don't need the DOF of course).
Sports shooters often want the shallow depth of field to separate their subjects from the background while wide angles also exaggerate the features of athletes. Also, to get the 1/1000 shutter speed that is almost standard in sports a wide aperture is needed. I use a 17-40 F/4 on a crop sensor for my basketball remote behind the camera and I find it's too narrow at the wide end and doesn't let in enough light (it's a poorly lit gym too though). The next step would be to use the 16-35 F/2.8 with a full frame sensor (and probably a strobe as well).
