Google’s Gmail has a nifty feature that lets you set up other mailboxes so they’re accessible in one place. Thus, instead of logging into one of my other domains’ mail client to answer email sent to “toothfairy@” (for kids who might want to ask about the business model), I can just do it from Google. Or, because my company’s mail gateway is set up to filter attachments greater than 3Mb, I can use it as an alias to exchange files created in this century, creating the illusion this came from my company so responses go where they should.
It’s convenience of one-stop email plus the privacy of keeping my real address separated from other things. Only, as Woodstock noted, is it’s not so private.
While the email does superficially go out with said alias, it contains your primary Google mail address. In Plain Text. Twice. The argument that this is an anti-spamming measure is lightweight – it would be much better implemented using an encrypted (or at least obscured) value. And not twice.