All the Wight Reasons?

[Inziladun]

A casual viewer of book-related threads here might have seen an occasional question regarding the nature of Barrow-wights.
One issue I don't recall seeing before is why the wights were on the Downs.

In the days of Argeleb II the plague came into Eriador from the South-east, and most of the people of Cardolan perished....It was at this time that an end came of the Dúnedain of Cardolan, and evil spirits out of Angmar and Rhudaur entered into the deserted mounds and dwelt there.

ROTK Appendix A

The Witch-king now had a clearer understanding of the matter. He had known something of the country long ago, in his wars with the Dúnedain, and especially of the Tyrn Gothad of Cardolan, whose evil wights had been sent there by himself.

UT The Hunt for the Ring

It seems pretty clear they were sent to Tyrn Gothad by the WK. But for what purpose? The Dúnedain of Cardolan were gone, dead or fled away. And if these spirits were the Witch-kings to command, wouldn't their services have been of more use somewhere else; specifically, Forcost, where he still had living enemies?