Why do you have to slow down here?

You have to slow down because of the motorbike. In German traffic rules, motorbikes are more vulnerable and less visible than larger vehicles, so you need to be especially cautious. Reducing speed gives you more reaction time for any sudden moves the motorbike might make and helps ensure a safer distance. This precaution helps prevent accidents, particularly on rural roads where the space may be limited and oncoming traffic is present. Always adjust your speed to suit potentially hazardous road users like motorbikes.