John wants to know what the 50mm lens is for. A great question, and prompted by a ton of bafflegab as you'll see. "Hi. My camera, a Nikon D3100 came with a 18-55 zoom lens. My girlfriend and I were told by the salesman that this was a good all around lens, even though it was a kit lens. He couldn't or wouldn't explain what that meant. Then we started to see some advertising from Canon on a "portrait" lens and thought maybe we should get one since we take pictures of each other mostly. We went back to the store, a big Canadian photo chain, and asked about this portrait lens. The same salesman told us that a 50mm lens was essential to our success and that we should have one. He pushed a Sigma 50mm lens at us, but the price was too high, nearly as much as our camera, so we bought a Nikon 50mm lens. We had to bring it back because it would not autofocus on our D3100. A different salesman said we bought the wrong lens, like it was our fault and then spent almost an hour talking about his photography and what he does. We got a refund and then bought the right lens online because we figured these people weren't helping.
Sorry for the long email complaining, but we have the 50mm and cannot tell why we bought it. Pictures look like the pictures from the 18-55 when we set it at 55. When we ask other photographers we meet, they cannot seem to tell us why we need it either. We feel like we were lied to. What is this lens good for? It is a Nikon AF-S 50MM F1.8 G lens. Thanks, John."
Well hi John and let me say how sorry I am to hear you got such a runaround. I work part time in a camera store (the same chain you mentioned, whose name I removed from your email) and I am really sorry to hear about your negative experience. I know exactly why the Sigma was pushed at you, or can guess pretty well. The Sigma is a great lens, well built and very sharp, but would be expensive compared to your D3100 kit. There are often salesperson incentives called spiffs, basically cash, to encourage certain behaviours, and some salespeople will hurl their integrity into the breeze for $5. Plainly you met one.
Your camera uses what the industry refers to as a crop sensor. All that means is that the sensor is smaller than the original 35mm film negative size on which most SLR cameras were based. Because the industry is often confused itself, this means that the 50mm lens that is optically designed to create an image big enough to cover the 24mm x 36mm of the original negative size, when you use a smaller sensor, you see less of the image circle. Does this hurt? Not at all but it creates the illusion that the lens produces more magnification than it really does. Why would you care? You probably don't need to. Short answer is that the 50mm lens has the look of a 75mm lens on film when used with a crop sensor.
Hence the first effort of mcmarketing. Back in the days of film, "portrait lenses" were typically between focal lengths of 85mm and 105mm. Even this is a silly argument because a good photographer can make a portrait with just about any focal length. This focal length produces shallower depth of field, and a very nice and very subtle level of perspective compression that makes faces more pleasing. This is completely subjective of course.
Canon decided to play a bit loose with terminology and refer to the 50mm as a portrait lens. Which it can be, when used well, but they did this because on their crop sensor cameras, it looked like an 80mm would on film which is sort of close to 85mm and then the mcmarketing took over.
The lens you own has a very wide maximum aperture relative to your zoom lens capability. When shot wide open, at f/1.8 it produces very shallow depth of field, meaning not much of the picture is in focus, usually the area between the tip of the nose and the front of the ear on a filled frame shot. The background is blurred creating nice separation, and this allows the marketing people to drag a mispronunciation of the Japanese word Bokeh into the fray. Bokeh refers to the look of the out of focus area and is even more marketing abused than calling the 50mm a portrait lens.
The wide aperture of your lens will also pass more light wide open and may allow you to capture images at lower ISOs or without flash, a limitation of the relative optical slowness of the 18-55 lens.
BTW a kit lens means that the lens comes in a kit with the camera. People who don't know what they are talking about often say that these lenses are poor quality. Optically they are very good, however to keep manufacturing costs down, the use of metal in the lens body is minimized in favour of high density plastics. For the most part this works very well but the lenses won't take the same amount of abuse as a "pro" lens costing 10x as much. It is true that low cost lenses, kit or otherwise, may exhibit more distortions than a pro level lens, but if you are sharing your photos online or on your smartphone, you'll never see these distortions. They only become obvious in large prints made from RAW and then edited images.
So don't worry about what you have. You are absolutely right that the 50mm lens produces images that look like what you get from your 18-55 at 55mm. In fact if you shoot with the 50mm lens at an aperture of f5.6 or smaller, they are going to be pretty much identical. The advantages your 50mm bring you are as mentioned relevant in low light and wide aperture scenarios only. Back in the days of film, it was said that a 50mm had the "look" of the human eye. This is mostly true from a relative magnification perspective. On a crop sensor camera it doesn't, but I don't think that this matters. Make pictures that make you happy, using whatever tool you like. The 50mm is a very good, optically fast lightweight lens. It doesn't have the focal length flexibility of the zoom but will produce lovely images if you do your job. Also film was one ISO only for the entire roll. Today you can manipulate the ISO for every image and with good sensors, low light is less a problem than it used to be.
You have a good camera with good glass. Go make images and have fun. Feel free to write again if you have more questions or need information on other lens options. Always remember that if you are in a store of any kind and you start to feel your personal BS detector begin to ping, you are probably right.