Why do nearly all meatloaf recipes call for bread crumbs?

[winter_spice78]

Is it supposed to act as filler or does it have another purpose? I'm curious because I have a tried and true meatloaf recipe that doesn't call for any bread crumbs or crackers and it tastes just like a traditional meatloaf (some people have said it's better) and makes the same amount of food.
Deble, here's the recipe:

2 lbs ground beef
1 egg
1 packet onion soup mix
1 c ketchup or bbq sauce
2 tbsp water
1/2 large onion, chopped

Combine all ingredients, adding the beef last. Preheat oven to 375 degrees. Line a glass baking dish with foil (makes for easier cleanup) and put the meat mixture in. Add more ketchup on top. Bake for 1 hr.