It's really a 'just in case' thing because legacy signalling electricity supplies do not have an earth. This is both good and bad; good in that it doesn't necessarily shut down if there is a fault, but bad because people can get zapped. With the non-compliant legacy systems getting older and older, faults are more and more likely. There have been several cases of maintenance workers getting a shock from cabinets, so this looks like a safety thing, especially as the item in the picture is in a publicly-accessible place.
Over time the signalling supply systems will be replaced but of course it takes time and money.