Why buy one? If you go to distrowatch, there are hundreds of distros that are begging you to use them for free.
Going back to what I was saying before, though, I have found that it has been easier to learn about the underlying nature of linux. This is because in windows, you don't need to be exposed to that for day to day working on the computer. Point and click - that's all there is to it most of the time. I think the stats say that 95% of people have never installed an OS, be it windows or whatever. You really don't need to know much to use windows for most tasks. In linux, there is rarely a day when I don't learn something new about the system through checking something on a man page or trying to tweak or fix something. The community is also more into knowing how it works as well. The majority of windows users just want it to work, whereas I think the majority of linux users are technical.