I just put my new undertray on this afternoon. The rubber strips seem to seal the airflow that passes through the lower radiator, and down through the vent. If you look behind the radiator, there is a curved piece of plastic which diverts the airflow down under the car. I think the back rubber strip should seal behind the curved plastic, to prevent any of the air from getting above the undertray (otherwise if airflow gets up in there, the undertray could start scooping more air and bending down which will be bad - ask me how I know). The front rubber strip doesn't seem to be as critical though.
Golden Gate BMWCCA - Car Control Clinics
'13 C4S/'12 328i/'04 GT3
Past: '09 M3 DCT/'03 M3 SMG/'95 M3... missing an E30 I suppose...