I also purchase jarred sauce and "make my own" with it. I pan fry meat of some variety (ground beef, chicken, pork, turkey or maybe some sort of chopped sausage), along with mushrooms, onions, and garlic. Often I'll add in any leftover veggies in the house (such as green peppers, peas, sweet potato or broccoli). When it's all cooked, I add a full jar of Classico brand Vodka sauce (a tomato based spaghetti sauce). Stir, heat and serve.
Why do I do this? Because I really, really like the taste of Classico Vodka sauce. Yes, I can (and have) made my own tomato sauce, and it's perfectly good, but it doesn't taste like vodka sauce. So, rather than try to figure out what it is that makes that particular brand of sauce so darn tasty, I just buy a jar and use it.
Also, I really hate chopping tomatoes. They're so squishy! And using diced tomatoes from a can often makes my sauces very runny. And I don't have a working food processor right now, because I broke my old one.
Never heard of "gravy" used to mean tomato sauce before this thread. And I LOVE the regular sort of gravy on my fries, with cheese curds, aka poutine (and have done ever since moving to Canada in the early 80's).