From the page:
"If the recipient is not online, it is possible to send to their Bitcoin address, which is a hash of their public key that they give you. ... This method has the disadvantage that no comment information is sent, and a bit of privacy may be lost if the address is used multiple times, but it is a useful alternative if both users can't be online at the same time or the recipient can't receive incoming connections."
It's funny that Satoshi seems to have considered sending coins to a Bitcoin address to be a secondary function; ie, just a "useful alternative", as he notes.