I used to mix it in with fried rice that'd been left sitting out for too long and turned really dry. Gives it some moisture and a vinegary edge, but probably not for everyone, since ketchup's trademark is stomping all over the subtle flavors of a dish.
When I was in elementary school, I'd dip my pizza crusts in ketchup at lunchtime. I still do that every now and then with Sriracha ketchup
Also, same elementary school lunch: on pizza days, they also used to give us a side of tricolor fusilli straight-up. Just plain pasta without even so much as a little olive oil. So, fuck it. It got blasted with 'chup.