Vintage flaring tool - the box is a clue to how old it is, but I don't know how old that might be. I am guessing 1950s...? Two bucks at an estate sale.
Sure I can use it! It's for making compression fittings on the ends of copper or aluminum tubing, such as a fuel line on our old car or old boat might need. With a threaded nut made for this type of fitting first slid onto the tube, this tool is then used to extrude the end of the tube somewhat, making the end of a trombone bell shape on the end. When then placed against an appropriate fitting, the nut is then slid up to the end and tightened. The flare becomes the portion that is pulled tight to the fitting, preventing it from leaking.