Why is tethered hover testing performed on helicopters?
The cable is attached to the helicopter with a tensionometer.  This can be used to vary the effective weight of the helicopter by varying the tension applied to the cable.

The cable length can also be varied from 0 height, to hovering in ground effect and  hovering out of ground effect.

This test enables data to be gathered quickly for the helicopter performance from minimum to maximum weight on the ground, IGE hover and OGE hover without the pilot having to control height (since the cable does this) and with quick and easy changes to the effective weight without having to add ballast to the helicopter for each weight.

This test is a simple and quick, albeit risky, way to gather data on hover and climb performance throughout the weight range.