Recently Lee published a comparison between the images from several different-sized cameras and there was no apparent difference. What's the point of buying a camera with a larger sensor then?Sensor size matters. Sometimes. You won't have the need for a bigger sensor until you know the technical side of it and find yourself in a lot of situations you would benefit from a camera having such a sensor.
The Sensor In a Nutshell
While there are "by-products" from the size of the light-sensitive chip, in general it's just a rectangular device that reacts to light. The size of the all-in-one captured frame depends on the size of the rectangle. The bigger the rectangular area, the more you will capture from the scene your camera is pointed at. It is like the window of your home. The bigger the window, the more you will see from the outside view. A crop sensor is smaller than a full-frame sensor. That's the main difference.
When You Won't See the Difference
If you photograph mainly in large spaces or outside using enough light, available or strobes, and have a modern camera, you probably won't feel the need to buy one with a big sensor. Big sensors are often less noisy at high ISO, but modern technology provides us with cameras where even micro four-thirds ones give us quite a good quality of image in not-enough-light situations. This is why I said the "window size" is the main difference, not pixel peeping.
Can't we use wider-angle lenses when we have a smaller sensor? Won't we see more? Think of using a wider lens is like squeezing your clothes in a small travel bag. You're getting your stuff in, but you don't want to put any of these clothes on without ironing them first. The same with wide lenses: they will give you more of the environment, but will squeeze the center part of it, because there's no place to fit the rest of the view and thus distorting the reality of distances between the objects in the frame. With a larger sensor you will be able to use a tighter lens, distances will be closer to reality, and you'll still see more than a smaller sensor.
Shallow Depth of Field
Most of you know, but let me say it once again: larger sensors don't give a shallower depth of field. A 50mm lens will give the same optical image regardless of the sensor size. It's the sensor that will "crop" part of the circular picture, given by the lens. All rectangular sensors do. If you have a tight shot on a small sensor and want to have the same with a bigger one, you have to get closer to the subject, because the "window" is bigger and it will show more than you need. This will naturally make the photograph with a shallower depth of field, because of the diminished distance from the camera to the subject.
Can You Simulate a Bigger Sensor?
Absolutely, but not always. If you photograph a tight interior you better not use a wide lens, but create a panorama out of several images. This will do good to the interior designer, to the viewers (who won't be fooled by the distorted perspective of a wide view), and to your portfolio. You can do the same for landscapes and even for portraits. It's a big tricky for the last case, but if you're budget is tight, you can get around that with some tedious post-processing work.
So, When Would You Need a Bigger Sensor?
In my opinion and experience, it's when you're constantly in low-light situations (although not such a strong argument) and in tight spaces and you don't want to use wide-angle lenses, but capture more of the view. If you have experienced more cases where utilizing a larger sensor was an absolute necessity, please tell us in the comments below.