my first real SLR that I bought was the 450D, so before the age of digital how did Canon differentiate its model line up feature wise? I mean back say year 1998 the determiner of IQ was the lens and film and the paper that it was printed on. My point is, why have so many sensors across a brand's line up, with 1DX sensor, 5D3, 5D2 and then the rebel line up of sensors, if all camera's just over a decade ago had the same ability to capture light correct? So what you pay for then is build quality, FPS, and metering etc.
It all bolts down to two item: Money and Requirement. Use existing Canon lineup as an example: On the FF, we have the IDx and the 5DIII with different sensor, different resolution, different physical size and different feature. So if you are into FF then you have your choice depends on your requirement and how much you want to spend. The same goes to APS-C with the 7D, 60D, T4i and Xsi. Then between the FF and APS-C that is another choice. In order to get the most out of FF execellent lenses are required. So everything is more expensive with the FF body. On the other hand, APS-C serves another purpose. For people that never make 20X30 inch print It is an excellent media. It is cheaper than the FF with less requirement on the lens (using the best part of image circle). Also for the "bird shooter", a shorter lens can be used.