Nice find. I checked mine and my guess is that it stores the wifi settings and the print jobs. For me it contains JOB1 to JOB7 and all but JOB1, JOB2 and JOB4 gives I/O error when I try to copy them. The QFM2202 is likely something created when the software checks if there's a write:able USB device present.
That mine refuses to read 4 out of 7 JOB-files when I insert it in my Macbook is a bit worrying. In fact, in the past I had several print jobs (sent to the printer over wifi) that stopped mid-print with the printer no longer responding. I thought a firmware update fixed that because it hasn't happened for a long time but now I almost always use the USB-cable to print so maybe there has been a problem with the usb stick all along. I doubt they use "special" USB stick that can only be read by their own printer.
Can you copy all the contents of the USB-stick or do you also get I/O-errors on some files?
EDIT: It works and prints fine without the USB-stick installed. I'm pretty sure it is just used for the Wifi-module and store jobs that are received through wifi. Likely the USB-stick works similar to the SD-card reader on the Jr. USB-transmitted jobs are stored internally and Wifi-transmitted jobs are stored on the USB-stick. I very much doubt it will help with the NFC-blacklisting but I will try replacing my (likely broken) USB-stick with a different (empty) one.
EDIT2: So I formatted a 16Gb USB stick, named it "USB DISK" and copied only the APINFO file to it. Plugged it in and powered up the printer. It automatically connected to my Wifi and I sent a small print job to it using Wifi. Right now it prints it. I will examine the contents later but I'll bet that I will find a JOB1-file and perhaps also a QFM2202-file.
EDIT3: JOB1 was present after print while QFM2202-file was not.
EDIT4: Did a full format with SDFormatter on the original USB Flash and then put back APINFO and QFM2202 files. Put it back in the printer and sent a print job with Wifi. All good. In my experience formatting the USB flash does not brick the printer. It can even be worth considering if you experience problems with wifi-printing while USB-printing is fine.