When Curly dies, Buck gets his second important lesson in how to survive in this wilder environment that he has been put in.
When Curly dies, it is because she was knocked off her feet. Once she was down, all the dogs of the pack jumped on her and she was soon dead.
So what Buck learns from that is a pretty obvious lesson, right? That dog died when she got knocked down, so don't get knocked off your feet when you are in a fight.
This lesson will come in handy later, especially when Buck fights Spitz for control of the team.