MMB Generation II speedometers e.g. have just one tiny turn indicator LED. These adapters are used to suppress "hazard mode" blinking in a reliable way, which would be the normal consequence if you connected the left and right turn signal circuit to one LED.

Of course the adapters will work with any other 12 V LED, too. They contain two diodes and will be connected to the turn signals of either side and the indicator light in the speed or dash board. They're waterproof, simple to connect, and much less delicate than two single diodes that are soldered and have to be wrapped waterproof anyway.