Though I'd imagine some do it because they can. Others may dualboot and put a Windows OS on because they have some software they need for work/school that they can't use mac for.
I'd also figure some do it because maybe they bought the macbook thinking they'd like macOS but then ended up hating it. So instead of taking it back and getting a Windows machine they just installed Windows on the macbook.
My husband has done it in the past because it helps him troubleshoot people's requests for help. Having a Windows OS allows him to guide them without having to remote into their computer. He also prefers the layout of Windows. But like @VirusZero mentioned he mostly did it because he could.