They're just like normal jumper wires, and serve the same purpose. It's just that they can save a few cents by splodging carbon on the board rather than using real wire, especially if some other part of the board has carbon on it (e.g. for touch button contacts).
The carbon jumpers have a much higher resistance than real wire, which is either known by the designers to be harmless, or may even be desireable (adding a pull-up resistor between two distant nets on the board without even buying a resistor = saving half a cent).
No, this has no relevance to hobbyist electronics, because it's all about saving fractions of a cent. The carbon is doing nothing other than being a very imprecise resistor.