It is that market of small utilities that puts money in the pockets of developers that care about the desktop experience, and that isn't exclusive of Apple systems.
By the way, there is an ecosystem of excellent small utilities developed open source for linux that are, in my opinions, superior to the ones on OSX, or at least, the one I work and am familiar with. Of course open source produces many useless tools, but I've also found unusable applets for mac, so...
Windows is of course out of the equation.
Living from donations isn't really a sustainable thing.