Phosphorus is a wallpaper browser and setter in the spirit of Nitrogen. It can be considered an unofficial port of Nitrogen to GTK+ 3. Phosphorus is different, however, in that it is plugin-based.
Phosphorus, at its core, is agnostic to how the selected image is applied as wallpaper. There are many different Desktop Environments that all have their own way of applying wallpapers. Soon, with Wayland becoming stable, every lightweight compositor will have its own implementation as well. It is therefore not viable for Phosphorus to implement all these different methods in its core.
That’s why Phosphorus leverages plugins for these tasks. The idea is that every Desktop Environment or every Wayland compositor has its own plugin to use its own backend. These plugins can be written either in C or in Python 3, and can be installed separately from Phosphorus.