Yea, the "tagging campaign" is to cookie the lead with Marketo tracking code. Such that if they then go on to do something on your website you then can see what they've done, ie they aren't anonymous.
I don't think a tagging campaign is necessarily required. You can ilicit the same response by sending out a great piece of content to your database for free. All they have to do is click onto a landing page & you're done. So why not offer them a great roi study or white paper or whatever.