Exporting the NFS root file system during test phase allow you to run functional or stress tests on your units without flashing anything on the on-board storage, saving time and giving you more control on the test process (think for example of a log file saved on the shared directory, that you can simply check at run time from your host machine). Moreover, the same root file system can be shared by more than one device, allow you to run multiple tests on more devices with a single root file system.