They usually aren't light, and they certainly aren't small, but the unique perspective a telephoto lens brings makes it an indispensable part of your kit, even when traveling. Let's review some shots taken with a telephoto to see how we can justify its size and weight.
Avoiding Distractions
In some cases, distracting buildings or people may be unavoidable. However, a telephoto can help eliminate those distractions and bring your subject, in this case the clouds and mountains, front-and-center:
While there is still a building this shot, many of the distractions have been eliminated. This single building helps frame the foreground and the mountains now loom large in the background thanks to the effects of the telephoto.
Compressing Different Planes
The compression effect a telephoto has can be used even more dramatically to tie together different planes in a scene; in the above example, the lake, the people on the bench, and the mountains behind are all in very different planes and are different sizes. However, we can bring them together and highlight all of them by stepping back and zooming in. When composing this shot, keep the hyperfocal distance in mind to help ensure that everything you want to be in focus is sharp.
This image really highlights how darktable's powerful masks can make a tough job much easier - we want to use a second instance of local contrast to really bring out the details in the mountains. Fortunately, we don't need to painstakingly create a drawn mask that follows every peak and edge. Instead, make a very quick drawn mask that roughly follows the mountains and then move the feathering radius and mask contrast sliders to the right to bring the mask to the edge of the mountains and stick to them exactly. See the before and after effects of these sliders below:
This works because the edge between the mountains and sky is well-defined and therefore easy for the mask contrast slider to distinguish. This contrast improvement makes a big difference in the final image:
Isolating a Subject Like The Human Eye
Have you ever noticed how you can watch something small far away and keep track of it? This ability to focus on an object, even one that appears small, is really useful.
Sometimes a subject is too large to really capture effectively, even with a wide-angle lens. In this example, we're on a large lake and can't really capture its essence because it's so big and we have nothing in the foreground to provide perspective. When you're in person, this is not such a problem because of this ability to focus on a subject (like one one of those distant sailboats), but in a photo this is missing. A telephoto allows you to pick out an object and feature just part of your large scene, making it more focused and thus more meaningful. While it's clear in the above photo that there are mountains around this lake, I think you'll agree that they're much more dramatic and meaningful in the telephoto shot below:
Once you've captured a shot like this, it may look rather flat due to haze in the area (you are, after all, compressing a lot of distance in this type of shot and demanding some degree of clarity). Fortunately, darktable can help. First, let's use the haze removal module to attempt to cut down some of the haze. We can make this even better using color balance rgb to increase the global vibrance, contrast, and saturation of the shadows and mid-tones:
Moreover, increase the brilliance of the mid-tones so the image doesn't appear quite as dark.
Next, we can calm down some of the contrast in the water by using an instance of contrast equalizer with a gradient mask and using the L and h channels in the parametric mask to roughly select only the water:
Finally, we can create a bit of separation in the image by shifting the greens towards cyan and the blues towards magenta with the rgb primaries module:
This effect is subtle but significant in helping distinguish the mountains from the lake:
With all of these adjustments, and the change in perspective thanks to the telephoto lens, it's quite clear how much more striking this image is than the similar shot using a wide-angle lens.
Conclusion
A telephoto may require you walk back a long distance from your subject, which is counter-intuitive. However, this new vantage point allows you to compose the shot in new ways not otherwise possible. This useful ability makes it worth the weight and bulk of carrying a telephoto in your kit.