More importantly, we can see part of the reasons why Luffy can take massive beatings. I can't believe he was taking such terrible beatings as a child. They always talked about Garp being rough on him for training but they never went into such explicit detail over what other things Luffy did that made him so tough.

Also now we can see why Luffy (and Ace) was already so strong when the series started. He basically became a pirate and an outlaw since his early childhood. That's pretty incredible and details like these are what set Oda several cuts above the rest.