Because the dry dock can be used for maintenance and repairs if there are no new boats being launched from the yard.I really don't understand why the barge is position between rail and dry dock. And why they need the dry dock in first place Don't they completed the submarine inside the construction hall.
Why not transfer from rail to the barge and slowly lowered the barge No need for dry dock like this video? Confused
It also saves space since you don't need to build rails extending all the way to the sea, and then having to find space for a drydock somewhere else.
The dry dock might also be used to do some additional work or finishing touches on the boat immediately after launch, without transporting the hull to another part of the shipyard.