Why would a mail courier ask for id when delivering a package?

Some items are illegal to be in the possession of people under a certain age, commonly 18 or 21. Some items maybe require parental supervision. For liability reasons, the mail-order company made an agreement with the delivery service to make sure that only a legal adult accepts the package. Some examples include pocket knives, ammunition, prescription drugs, airsoft rifles, etc. Some packages must be delivered only to a specific person. These packages may contain sensitive materials, such as legal doents, medical records, or new social security card, etc.