The reason should be pretty obvious for a lot of people by now, I did it because of proprietary software. Now, what do I mean by that.
So I've been using ThinkPads for a few years now and almost from the beginning I was annoyed by the fans, not that they're particularly loud or something, the fan profile is just fucked up real bad. Engineers who wrote the software controlling them on the older models should be ashamed. The thing is you can't change the fan profile, at least not in the BIOS/UEFI, there's no official way of doing that, afaik. What you have to do is use third-party software.
I've had quite a good experience with thinkfan, but at times it felt too complicated for what I wanted to acomplish, so after some time I decided to put my programming skills to a test and write my own fan control software. I can now say that I succeeded.
stfc, which stands for simple thinkpad fan control, is as the name suggests a simple piece of software that allows you to change the fan profile on your ThinkPad laptop. It's Free Software, written in C and takes after suckless tools (it's configured during compilation etc.). The source code is available here. You're free to do whatever you want with it as long as you abide by the license, in this case GPLv3.
What I wanted to say, though, is that I wouldn't have written it if the BIOS/UEFI firmware on my machine was Free Software, because then I would be able to just change the fan profile by modifying it instead of relying on 3rd party code (in this case it's mine but that doesn't matter) and we don't know for how long the thinkpad_acpi module will be able to alter the fan profile. Lenovo might just turn that feature off in their next generation of laptops.
That's it; stay safe, write Free Software.