It could be for return packages to earth, but with the rail gun already in place, that seems somewhat useless.

On the other hand, it would be difficult to aim the rail gun, whereas the space tether would be somewhat easier to aim.

In this scenario, we're just missing powering the rail gun self-sufficiently for a tidy operation. So a method to convert that angular momentum into electrical power for our rail gun would be the final step I think.

Fly wheels, gears, electrostatics in the tether. All equally undeveloped at scale as our tether except in principle so maybe the tether makers in the scenario already had an idea in mind.

The question is...will they settle on a steam turbine in space somehow lol?