It is a measure used at some airports. It is supposed to increase the situational awareness to pilots, as they then know that vehicles are cleared onto certain areas of an airfield, as they hear the ATCOs transmissions, whist avoiding that potentially non-RTF licenced ground staff do not block, or transmit on, a licenced VHF frequency. Also may be relative to the types of frequencies and equipment available at the airfield for the ground staff.
It is not ideal, as all staff adequately trained, operating on the same frequency would be the safest operational situation.