Usually done post engine change, In reference to leak check an obvious one will be discovered. Also on most jets FF will not register properly due to air in the system that needs to be purged. I commonly perform the process after changing engine fuel components to prevent fluctuations on start that may indicate an abnormal start by indicated fuel flow. The Fuel in the tail cone is no big deal as long as it is followed by a lengthy dry motor to purge it before engine start.
I have remedied FF issues on engines that have sat for a long time with this process.