For the same reason you would trust a security consultant to attempt to penetrate your network's defenses and find deficiencies. Any of your employees could bring in a similar device, what are you going to do about it?
With a real one in place, you can come up with a strategy. Maybe your IT team comes up with regular sweeps for cell signals, then the security consultant decides that the cell signal is only active during short periods of time in the day. Perhaps you secure your power sockets, and then the consultant uses the fact that your janitor needs a plug to vacuum and doesn't find a power strip suspicious.
With a real one in place, you can come up with a strategy. Maybe your IT team comes up with regular sweeps for cell signals, then the security consultant decides that the cell signal is only active during short periods of time in the day. Perhaps you secure your power sockets, and then the consultant uses the fact that your janitor needs a plug to vacuum and doesn't find a power strip suspicious.