History! That's what makes postcards valuable to this collector. Valuable in the sense that I appreciate their importance and often scarcity. As an investment, forget it. Get into art, that's the best vehicle (if you know what you're doing).
This is one of my favorites. A picture postcard sent from a National Guard soldier on the border with Mexico during the Pancho Villa Expedition of 1916. Columbus, NM was the site of Villa's raid a few months before.