Why must you not drive extra slowly for no good reason?

You answered everything correctly. Driving unusually slowly without a valid reason is prohibited because it can block or slow down other vehicles, disrupting traffic flow. It can also surprise drivers behind you and increase the risk of rear-end collisions. Additionally, if you drive much slower than necessary, it may provoke other drivers to overtake you in unsafe ways, raising the risk of accidents. In summary, German traffic rules require you to adapt your speed sensibly to the traffic situation and not endanger or inconvenience others by driving unnecessarily slowly.