In the absence of external torques, the total angular momentum of the Earth as a whole system must be constant.

We assume that the molecule is in a homogeneous (no external force) and isotropic (no external torque) space.

A Spacecraft needs an attitude control subsystem to be correctly oriented in space and respond to external torques and forces properly.

The Euler top describes a free top without any particular symmetry, moving in the absence of any external torque.

However, external torques on the spacecraft may require a gradual buildup of reaction wheel rotation speed to maintain the spacecraft in a fixed orientation.

Thus its angular momentum would be unchanged, unless an external torque were to be applied.

Angular momentum is conserved in a system where there is no net external torque, and its conservation helps explain many diverse phenomena.

So requiring the system to be "closed" here is mathematically equivalent to zero external torque acting on the system:

In the absence of an external torque, the angular momentum of a body remains constant.

The higher the angular momentum, the greater the resisting force of the gyro to external torque (in this case more ability to cancel boat roll).