Why are flat copper cables with holes used for high voltage ground?
Flat, semi-rigid metal strapping is sometimes used to electrically bond pieces of equipment together when they aren't expected to move significantly with respect to each other. A flat strap is easier to bend into shape than a round wire is, and doesn't require any specialized termination hardware. The holes are there so that it can be fastened to the equipment at each end with a screw or bolt.


(source: nibco.com)