In 2367, the host of Odan used this tool to scan his symbiont. The symbiont reacted by pushing outwards beneath the skin. (TNG: "The Host")
It is unclear what this tool actually does, how it stimulates the symbiont, or whether it has healing properties. The script of the episode states: "Odan takes a device from the counter, aims it toward the bulbous protuberance, and envelops it in a spray of light. The growth throbs there for a moment, then begins to recede again. Whatever it is, Odan has just fed it, and it is content once more."