The original intent of the referral tracker is to give you credit for anyone who comes to HP via your subdomain. If they sign up to HP, then you get 10% of whatever they earn on HP (from the HP 40%, not your side, so you lose nothing). This is why the referral trackers are under the Earnings tab.
I have used them for that, but mainly I use referral trackers to see where hits are coming from. It helps me gauge which social media work best for driving traffic to my hubs. I create a referral tracker with a meaningful name, such as "FBPosts", go to the bottom of the hub I'm promoting and click on "link to this page". You'll get a popup that has the list of referral trackers. Choose one and it is automatically inserted into the URL. Copy/paste and voilá! Anyone who comes to your hub via that URL will get counted under that referral tracker.
My own opinion is that this method of tracking is much more useful than trying to track actual referrals for HP sign-ups. Thus, Referral Trackers menu item is more suited to be under the Hubs tab in your account dashboard where Traffic Sources already is.