Some people collect, but for many they are functional purchases. If you have to spend 8-12 hours a day typing on something, you really do want the perfect keyboard. Modern keyboards span from absolute crap to great, but there are a few vintage keyboards that incorporate features that you can't find on any modern keyboard (the best way I can explain this is a car built in the 70's compared to a car built in the 90's), and some people love them. Since they are no longer made, and time reduces the number of functional units that are available, they tend to increase in price with time. For some people finding just the right keyboard for their typing style is the difference between optimal performance and productivity, and aggravating a persistent medical condition and being in constant pain.
I say this as someone who uses a specific model of keyboard that was made in 1991 and uses two adapters to interface with a modern computer.