They don't have to follow it but leading decisions (Grundsatzentscheidungen) still influence how a lower court judge might interpret the same law. Based on providing legal certainty instead of stare decisis though. It's not a must, more of an... aid
Still - a judge on another case will probably factor this one into their decision, but if they have a radically different view or the case is sufficiently different, they could just as well come to a different conclusion.