It doesn't feel to me like much of a gain over having said team expose a specific default self registration function. Then that act can be explicit rather than action at a distance on some dependency defined global state.
I think this pattern in the standard library is a mistake.
But one uses a strange construct to save a single line, loses the ability to control order, and encourages people to use globals that they can't control.
I very much dislike import side effects in any language. Im a lot happier where thing.Register() is still forced to happen in the main function.
Still, I can understand it for some components like loggers to not add boiler plate to every library. However, I was very uneasy to see that enabling gzip decompression in a gRPC server is done through a magic _ import. You have to initialize the server anyways, so why not just make it an explicit function argument?
thing.Register() may register a route that conflicts with yours and it will never be matched in this case if declaration order was taken into account.
You may discover this too late when you either have missed important calls on that thing or when requests intended for that thing are causing unintended effects on your first declared route.
I think this pattern in the standard library is a mistake.