Length means nothing. Weight's important for both breeders and pets.

Breeders already know why they need to monitor weight, but it's useful for people who just keep BP's as pets. Situations like fasts where the snake hasn't eaten for three months tends to cause some people to panick and think it's losing large amounts of weight. A reguarly weigh-in can keep you from panicking. It's also always good to have the weight records when you go to the vet in case they request it.