its partly for taste - cooks better IMO. I find pure lean sirloin tastes strange and pure turkey burns. The 50/50 combo works well for me. I usually get about 1.5lb turkey / 1.5 lb. sirloin, crack an egg, Worcestershire sauce is awesome so i add that, dice up a red and green bell pepper, garlic, onion, garlic powder, pepper, salt, if you have liquid smoke seasoning thats good..... its really pretty easy I just look around and throw stuff in without really measuring usually. Here is a good actual recipe - Invalid Link Removed - but I dont really follow it to a T.