I use ketchup for one thing, and one thing only: french fries.
Anything else I have tried eating it with makes it taste like I am simply eating ketchup, and it just seems wasteful and pointless.
But I know people who eat it with everything. Different strokes...
I use ketchup for my home-made 1000 Island Dressing, sloppy joes, cocktail sauce, and sweet and sour sauce.