Ketchup is much sweeter and more acidic than a standard tomato sauce, but it’s actually a suitable substitute if your recipe only calls for a small amount of sauce.
Ketchup’s tangy and slightly sweet flavor works with many recipes and provides a hint of tomato flavor, much like a spoonful or two of tomato sauce would.
It may not be your mother's marinara, but ketchup's versatility makes it suitable for many cuisines, and its thick texture can also help bind sauces and condiments together.