Importance of Duffel bag

Alistair accidentally leaves his duffel bag at the Lyceum Theater the night he meets Mary when London first comes under attack by the Germans. Alistair rushes off to organize his men to help pull wounded people from the rubble. Mary brings the duffel bag to him at the station, using it as an excuse to see how he is doing. This is the last time they will see each other until Alistair is discharged from the military after prison.