[jjeaff]

Probably just the fact that pedestrians and other cars can hear you coming if you have loud pipes.

Motorcycles in particular have the danger that if a driver in a vehicle doesn't see them, they can get ran over. Being smaller, they have a tendency to easily hide in blind spots or just not be noticed at all because the driver is expecting another large vehicle.

So many motorcycle riders use flashing headlights and loud pipes to announce their presence.