People will come back for more if you have something scarce on your site on a regular basis. Why people still fall for this is beyond me, but they do. You say to yourself - it's only a sales gimmick. Yes, it certainly is. It's a calculated gift as well.
Compliance goes up by a healthy margin if a free gift is included.
In our society this rule requires that one person try to repay what another person has provided. This might be a little overused on web sites these days, but applied with finesse it works well. Especially if the free gift has at least a perceived value. The obligation to repay lingers, and kicks in when you ask for the commitment. This is evident in sites offering a high quantity of very hard to find and valuable information.