Many email etiquette guides caution against using the blind copy, or BCC, feature of your email program. One article on Business Email Etiquette calls the practice "impolite and sneaky". Another, from Net M@nners.com, allows that there are both polite and impolite reasons to use BCC.
The most common and acceptable use of BCC is with mailing lists, says this article on LifeHack. You send a message to many people without revealing all their email addresses to each other. This protects their privacy.
Recently, I've begun using it for a different purpose. I'll include my own email address on an outgoing email, but in the BCC field so it doesn't show on the message that others receive. This plants a copy in my Inbox so I can easily drag it to my @Waiting folder as a tickler to follow up if I don't hear back. It's easier for me to stay in the Inbox and drag the new message, than to switch back and forth between Inbox and Sent. This is especially the case if I'm accessing my email from a client's network via Outlook Web Access, in which switching folders is a bit more work.