They don't run an extra cable though, they just buy credits and if you're lucky the profit doesn't go to the parent fossil energy company. It's still better than old energy firms but it's not as if the coal plants run any less hard when you turn on your bitcoin farm with a green contract instead of a fossil one. It's just a matter of where profits flow, and also some public perception. (E.g. Climeworks says that individual customers help show investors that that's a market for it, which helps them do more good than purely the CO2 you pay them to remove.)
Regarding this "fossil parent" thing btw, list of checked companies: https://www.robinwood.de/oekostromreport (I'm with Green City Power because they don't only go the easy hydro route but also build out solar -- iirc, it has been a while). Switching is a matter of signing up with the new provider. Nobody needs to come by, power doesn't go out at all, you just tell them what the meter says on date X and all is good.
Regarding this "fossil parent" thing btw, list of checked companies: https://www.robinwood.de/oekostromreport (I'm with Green City Power because they don't only go the easy hydro route but also build out solar -- iirc, it has been a while). Switching is a matter of signing up with the new provider. Nobody needs to come by, power doesn't go out at all, you just tell them what the meter says on date X and all is good.