The buddy list has at least one other function:
Unwanted PM Filter
This works like the ignore list, only backwards. With the ignore list, you specify people you DO NOT want to receive messages from, while with the Buddy list you can specify people you DO want to receive messages from. By checking the "Receive Private Messages only from Buddies and Moderators" in your profile you can ignore most private messages. As the User CP says, messages from those not on your Buddy list "will be told that you have disabled private messaging."
Note: You cannot block messages from forum staff members in this way