To help you to save disk space. The file parameter next to the -r is the one that has the weak packets. For this file only the bytes until the end of the IV (Initival Vector= first 4 bytes after de 80.11 header) must be present. So in order to save space you can use --caplen for the capture session, to just save a custom number of bytes for each packet.
The other file parameter is the one that has at least 10 wep data encrypted packets for key testing purposes. All the bytes of these packets must be logged, because the CRC is at the end of the payload and is needed for the key testing procedure.
But if you dont mind the diskspace you can just use --caplen 0, so all packet's bytes will be saved. Then you specify the packets twice for the FMS attack, one for the weak packets and another one for the verifications packets.
That's why in the example we put the file twice
I thinks it is a good idea