After clarification with the product manager, we inform you that this filter is not compatible with the WD 2.400 M. However, this model should be: Kärcher Cartridge filter

The cartridge filter WD/SE (manufacturer product number 2.863-303.0) is compatible with the Kärcher A 2054 ME. It is therefore suitable for use in this model.

You use the filter if you use the appliance as a wet vacuum cleaner. If you use the appliance as a dry hoover (hoover), you will need the bag.

No, the filter is optional to use the unit as a wet vacuum cleaner.