Aside from the fact that I love the game, my copy has sentimental value as I bought it new bitd. I haven't used the kisado I got with SFII' bitd for years now, but I can't bring myself to get rid of it for something as easily obtainable as money, which I have a steady stream of. I also love the TG-16 and PCE. If everything Turbo/PCE related became valuable, I wouldn't sell it all either. That's not why I buy games. That's not why most people are buying Magical Chase for collectors prices today. People who buy games as an investment and don't plan on keeping anything, but happen to currently own MC, are the ones you should be asking this question of. Otherwise, why don't you just ask why anyone bothers with this forum at all (other than for buying/selling).
As for the sealed games I own, I plan to open almost all of them and I buy them so that I can experience opening a new game and only having it worn and abused by myself, just like games I bought new bitd.