Usually the best testbed for engine development or if you want to quickly play around with an engine feature.
Also very useful in mod development due to its minimalism,
which allows for a very fast loading time and also helps ensure that
your mod works even in a minimalist environment without bloaty mods such as default.
Devtest also provides a few introspective facilities (e.g. node editing tools etc.) and example content (nodes, entities etc.) that aid in engine and mod testing.
That said, when you're developing a new game for players, you should under no circumstances base it on devtest (except if your game is intended to serve as a replacement testbed or perhaps a more specific mod testbed).