In Dortmund you can almost read the occupancy calendar off the BVB fixture list and the dates at the Westfalenhallen: on those days a large number of out-of-town guests look for a room at short notice, often from their phones. A house near the city centre, in the Kreuzviertel or in Hörde that shows up right away with a clear, mobile-bookable site gets the enquiry — instead of losing it to a portal.
During these demand peaks every commission-free booking counts twice, because the rooms are in demand anyway and need not be filled through an OTA at 15 to 25 percent commission. Combine that with the regular business travellers from Aplerbeck or Hombruch, and you build a base of direct guests that keeps the earnings in your own house.