Kivy
Kivy is an open-source Python framework for developing multi-touch applications with a natural user interface (NUI). It is cross-platform, running on Windows, macOS, Linux, Android, and iOS, and uses OpenGL ES 2 for hardware-accelerated graphics. Kivy includes a rich set of widgets and tools for building interactive applications, particularly suited for mobile and desktop apps with custom UI designs.
Developers should learn Kivy when they need to create cross-platform applications with custom, touch-friendly interfaces using Python, such as mobile apps, games, or data visualization tools. It is especially useful for projects requiring rapid prototyping or deployment on multiple platforms without rewriting code, as it abstracts platform-specific details and provides a consistent development experience.