I have always wondered why people put oil or butter in a pan before browning ground beef. It doesn't make sense since you end up having to drain the grease in the end anyways. What is the purpose of the oil or butter? Is it used for pans that are not non-stick? Flavor?
Transfer of heat. First you need a hot pan (sprinkle some drops of water on the pan, it should sizzle). Then you add a drop of oil and swirl it around. When the oil forms a striated pattern, it's hot. Then you put the meat in.
The hot oil helps to transfer heat from the bottom of the pan to the meat. You only need a spoonful of oil.
If the ground beef is very lean then adding a small amount of cooking oil to the pan is necessary. Otherwise, ultra-lean ground beef will burn and stick to the pan before the center of your burger is even done. Additionally the burger would be extremely dry and unappetizing when cooked.
After it is done squeeze out excessive grease with napkins or paper towels before consuming.
Saturated fats and unsaturated fats will form a bond producing triglycerides. Rinsing the beef with hot water, practically makes your 73/27 beef into a 90+/10- beef. It's healthier and much more cheaper per pound! With that said, butter may make it Brown and taste better, due to the dairy components and thus adding more saturated fats.