What Is The Benefit Of Pumping Nitrogen Gas Into Seawater That Is Stored In Steel Tanks?

Nitrogen is a cheap gas that's relatively non-reactive (but not inert).  It would displace any oxygen and reduce the corrosion of the tanks.  They may also be using it to keep the seawater stirred up, without causing
any change in pH or oxygenation.  If they simply used air, that would
increase the dissolved oxygen just like we do with air bubbler stones in an aquarium.

Its displacing the amount of oxygen to reduce chemical reactions in which would result in corrosion... Hence the steel tanks (steel is reactive).