Gas Line? Vent Line attached to valve?
The flare fitting seems to indicate that it is gas supply tubing that leads to an appliance of some sort. It must lead to a gas dryer, gas cooker, gas grill, gas water heater, or gas fireplace (or did at one time). The valve is OFF, so there should be no harm disconnecting the old line and using the line to supply whatever you want; obviously whatever it leads (or led) to is not in service.