I hate single color icons. For me Tango icon theme is peak of usability, anything else is downhill from there.
These icons have outlines. On the plus side, they look decent on any color background (though Humanity icons have too light-colored borders, which can make light icons hard to see on light backgrounds). On the minus side, the outlines look noticeably blurry when rescaled instead of picking the nearest integer size.
Monochrome icons are less damaged by resizing. However, they can become essentially invisible black icons on a dark background (or vice versa), so often come in light/dark variants. Apparently KDE has an extension to XDG icon themes called "FollowsColorScheme", where the renderer will automatically recolor -symbolic icons. (Documented at https://github.com/lxqt/lxqt/issues/1181 )
They are mostly used for aesthetic reasons. It wasn’t that long ago that icons were expected to use color and shading. For usability’s sake, we should hope some version of that trend returns.
Imagine for example a piece of software with a settings menu which maybe gets used once per user, if ever, and it is only for technical stuff (think: assign more RAM and change V-Sync method). For that use case, a simple, subtle icon which is perfectly findable for the target group in the rare case it is needed is perfect. A colorful, detailed icon might disproportionally distract and thus diminish user experience.
Point is - use the right tool for the right job, and thanks to the author for providing a specific tool!