Size Does Matter
All else being equal, a lens with a faster maximum aperture will be bigger and definitely heavier than a more modest one of the same focal length. So why do we all want to carry them around?
There have been various answers to this question put forth over the years, but clearly on possible explanation that has to be near the top of the list is simply the fact that "professionals" use them. Big lenses with fast glass are a form of status symbol among aspiring photographers. When photographers gathers around the scenic overlook and whips out their cameras, the one with the biggest lens wins. Clearly, they're more serious about their craft that the other shooters there that day. Everyone else looks at them with a touch of envy, wishing they could afford such nice lenses. Don't deny it, you know there's at least an element of truth to this answer.
But let's turn our attention to more practical explanations.
A wider aperture creates images with a shallower depth of field. If your objective is to shoot dreamy images with only the minimum of compositional elements in focus, you need a big, fat lens with a wide aperture. Portrait photographers crave extremely wide apertures to control how the background gets rendered. The goal is to have the subject in focus, but not the backdrop behind them.
But most outdoor images tend toward the opposite extreme. Ever since the days of Ansel Adams, Edward Weston, and Group f.64, landscape photographers have worked to create images with maximum depth of field. Everything from the extreme foreground to the extreme background was to be in focus to be successful. That sort of insistence isn't as prevalent today as it was back then, but it's still common. I've heard judges at photo contests complain that an image isn't completely sharp when clearly it was never the photographer's intent that it should be. Depth of field is a creative control and neither end of the extreme is right for every occasion.
But if it's common to at least stop a lens down somewhat, why would anyone care about the maximum aperture of a lens? That is, if they never actually shoot at that aperture, why does it matter?
Here's another piece to the puzzle. A lens with an aperture of, let's say, f/2.8 can let in twice as much light as a lens just one stop slower, at f/4. That's what aperture f/stops mean, after all. Double or halve the f/stop and you change the amount of light the lens can transmit by a factor of two. So a faster lens lets in more light. That means you can shoot in low light situations when a slower lens would be, shall we say, left in the dark. If you press the shutter release and lens aperture has to stay open too long for an exposure to be recorded, your subject could move and end up blurred in the resulting image. Shoot with a faster lens, and you can freeze motion even under dim lighting conditions.
But the sensors used in modern digital cameras have a much better ability to capture images in low light regardless of aperture. We have the ability to push the ISO to levels that would have seemed unimaginable just a few short years ago. Not that long ago, shooting at ISO 800 or above was considered an act of desperation. If you couldn't get the image any other way, you might be willing to accept the increased noise levels that came with higher ISO settings. It was a compromise at best, and if you had a fast enough lens you didn't need to compromise. But ISO 800 is no big deal these days. Many cameras can create perfectly good images at ISO 3200 or even higher. At ISO levels that high, you don't really need a fast lens to keep shutter speeds fast and avoid subject blurring.
Have we just not realized we no longer need fast lenses to get good images in low light? No doubt there is at least an element of this in the mix, but by now it should have started to sink in that big, fast lenses aren't really needed anymore if that were all there was to it.
It's generally understood that the "sweet spot" for most lenses is near the middle of their aperture range. Perhaps having those fast f/2.8 apertures but not often using them means we'll get better results at the more modest f/5.6 or f/8 apertures we do generally use. That seems like a bit of a stretch, but it is at least partially true. A lens that only goes as wide as f/5.6 probably isn't as sharp as an f/2.8 lens stopped down to f/5.6.
Some of this difference though is based simply on the fact that the f/2.8 lens no doubt uses better quality glass than the slower lens since it has to in order to still be acceptably sharp wide open. Most Nikon fast lenses have at least one ED glass element. Fewer mid-aperture lenses do. But paying for and carrying around wide aperture lenses purely for the ability to stop them down before shooting seems a bit far-fetched. If there weren't still some other reason, Nikon would make more f/5.6 lenses with ED glass and offer fewer f/2.8 lenses.
In my experience, one of the most compelling reasons for buying fast lenses even when I do often stop them down before shooting has more to do with me than my camera. Even though cameras can now create great images at high ISO speeds even with mid-aperture lenses, my eyesight still has the same difficulty seeing well in the dark as I always have had. I'm not Superman, and I can't see in the dark. A lens that opens as wide as f/2.8 allows me to see what I'm shooting better than one that only opens to a something less than f/2.8. And if I can't see my subject and the scene around it, I can't compose well. Sure, I may end up with the subject in there somewhere, somehow, but I'd be unlikely to create the best image I can when forced to look through a dimly lit viewfinder. Even if I do stop the lens down to shoot, it will automatically be wide open with I compose that shot. I like that, and it remains one of the biggest reasons I'm willing to invest in fast lenses.
That's not to say of course that I mind showing everyone else up when it comes time to compare lens size.