This is probably one of my favorite features. As an enthusiast photographer I always look for useful camera features. Now, the wide-angle and telephoto isn't a new feature by any means, but we can see that some companies decide to go with a second monochrome sensor to enhance the low-light performance of the primary camera. I personally prefer having this type of setup, as it provides me with more ways to capture unique and interesting photos.
Some people say that you can just zoom-in using the digital zoom, but digital zoom impairs the image quality by a lot. Using the telephoto camera means that there is no negative impact on the image quality as the zoom doesn't rely on digital processing (image enlargement and cropping in this case) to do that magnification job, it's all done optically.
The main downside of the telephoto camera is that it isn't great for low-light photography due to slow aperture lens and small 1.0-micron pixel. However, I personally find myself using the telephoto during the day when there is enough light from the sun, so it isn't that problematic for me.
The first camera is a wide-angle camera that comes with 1.4-micron large pixels and a dual-aperture lens. A dual-aperture lens means that the camera can automatically switch between an F1.5 and F2.4 aperture based on the light in the scene. To prevent overexposure in bright scene, the camera will automatically switch to the f/2.4 aperture which means less light will pass through the lens. When shooting in the afternoon or in low-light situations, the camera will switch to F1.5 aperture, which means more light can pass through and making sure your low-light photos will look at their best.
Having an option to choose the field of view (wide or telephoto) is a great advantage for anyone who understand the benefits of having these two options. This means that you can come home with more interesting shots and have less limitations over the creative possibilities when shooting outdoors and in different lighting conditions.
Overall, I see it as a great advantage for the Samsung Galaxy S9+ over the S9 (which has a single rear camera) and over other phones that offer a different implementation of the dual-lens camera system.