Dutch ovens can go both in the oven and on the range, so they're a fair bit more versatile than a cheap roasting pan and allow you to get by with fewer pieces of cookware cluttering up your kitchen.
They also tend to be more durable. Our Granite Ware stuff inevitably has the enamel chipping off after a few years. By contrast, my mother in law's dutch oven is at least 50 years old and still going strong. You don't have to go full Le Creuset, either - there are plenty of quality cast iron dutch ovens that only have to last twice as long in order to be cheaper than the cheap roasting pan.
They also tend to be more durable. Our Granite Ware stuff inevitably has the enamel chipping off after a few years. By contrast, my mother in law's dutch oven is at least 50 years old and still going strong. You don't have to go full Le Creuset, either - there are plenty of quality cast iron dutch ovens that only have to last twice as long in order to be cheaper than the cheap roasting pan.