Mail addresses are universal identifiers and should be used to identify keys. However, sometimes there are several encryption capable keys and the owner does not want to receive mails encrypted to one of these keys (e.g. because it is used only for internal purposes and available only on an offline smartcard.
A dedicated notation could be used to exclude such a key for key searches for encryption. A gpg option would then be used to allow it anyway. This would be a kind of extended key usage.