If the ground beef is too lean, it will fall apart while cooking. If it's too fat, the burger will shrink like crazy. Look for 70/30, or up to 80/20 ratios. Any less fat than 20% will be too lean.
That said, however, leaner ground beef has a more pronounced beef flavor. The fat tends to dilute the flavor. But the resulting patties are dry in texture and mouth feel. This is where the egg comes in. To keep hamburger from shrinking, add 1 large egg per pound of hamburger. You won't be able to taste any difference, and the pattie will be juicer. This is for fatty ground beef.
But, that same egg will bind together ground beef that is too lean, and make it juicier as well, and again will add no flavor to the beef.
A little trick to making patties. Try to make the pattie round, and with smooth edges. Make the center thinner than the outer rim. The burger will always shrink a little toward the center, making it bulge if the initial pattie is the same thickness all the way across. That bulge can make it so the hamburger is undercooked in the middle, and overcooked on the outside. By making the center thinner, you insure that the pattie will be cooked evenly everywhere.
If you add flour, or breadcrumbs, or chopped veggies, then you are creating a Salisbury Steak, instead of a burger. It can still taste great, especially with gravy. It doesn't stay together well though, and is very hard to grill as it easily falls apart.
After I'm through with the properties of flour and fat in my blog, I'll go into the subject of meats more thoroughly. Don't be afraid to try different things with your burgers. You might find a seasoning that your really love. me, the close it tastes to steak, the better I like it. A little salt and pepper, and cooking over charcoal makes me the burger that I crave.
Seeeeeya; Goodweed of the North