Because cycling at night is dangerous.
Because dynamo powered lights rock.
With this little mod, you will be safe while stopped at the crossings: its supercap will keep the led(s) lit while you wait for your turn.
The circuit on the left uses a clever technique to limit the current and increase the voltage: a charge pump. My only regret is that it uses a linear regulator to keep the voltage at 5V.
While this is obviously inefficient, it works and with a hub dynamo you will probably never feel the difference.