If I understand correctly, a spool is a temporary storage location. Traditionally, the mail is stored in a "mail spool", a mailbox in the
/var/spool/mail directory, where users are expected to pick it up. Therefore, according to this view, once the mail has been read it should be deleted or moved elsewhere — most likely somewhere in the user's home directory.
This way of handling the mail doesn't seem very efficient to me. And where does a program such as
procmail fit into this scheme? If you have an MDA that automatically stores your mail into folders in your home directory, then why keep a mail spool at all?