Sort:  

It's a balance of what you want to do, versus what you want to let the camera do. For example, for these ones I used aperture priority. I set the camera to F2.8, and the camera chose the shutter speed and the ISO. This is what gives you the least depth of field and makes the background blurry. In contrast, using a completely automatic camera will attempt to get everything into focus. Lenses give you a lot of flexibility. I've seen people on safari trying to capture animals with iPhones and iPads, and the end result is an animal that looks like a speck on the photo. A 400mm zoom in that scenario, you get get much 'closer' to the animal without disturbing it. For travel, a really wide angle lens is good, because you can get a lot of scenery into the frame, and closer to your subject than the throngs of people with their cell phones.