It can be usefully used to easily test the display server or custom configurations, given that it loads quickly and doesn't require any additional libraries.
Twm was renamed Tab Window Manager (from Tom's Window Manager) when it was adopted as X11's default window manager back in 1982. It was left behind many years ago but still remains as the foundation for every window manager that has followed.
Most aspects of twm's interface operate differently to those of UIs more commonly used in computing. This implies that reading the manual will usually be a prerequisite prior to its use.