My view on it:
Its an ops procedure if both gps are inop / GPS primary not active, to have a nav accuracy as high as possible.
At takeoff the FM position is updated to the runway threshold position as stored in the database.
A takeoff update requires that the takeoff runway is part of the flight plan. This provides the most accurate position update.
If the takeoff run starts at an intersection, with gps primary not active, enter a takeoff shift on the PERF TO page to refine the takeoff update.
An accurate takeoff update ensures a more precise aircraft position during departure.