But for my money, the thing a double boiler does best is re-heat. Things with a high liquid content (leftover spaghetti with sauce, say) heat up without drying out. Cooked vegetables stay crisp. Even something like a grilled chicken breast gets hot and actually gets juicier from the process.
Unlike a microwave or a toaster oven, the food reheats evenly and thoroughly. Granted it takes longer than those other methods, but it's definitely worth it if you've got the time.
Today, I had the time. And right now that leftover rigatoni with turkey meatballs and broccoli is looking mighty good.