What's the point in tracking?

Say you are promoting a Clickbank product via your website, e-mail list and pay per click (OK mainly only advanced marketers will be doing all three right now). You make 20 sales of a product, but you have no idea through which media you made the sales.

Highly annoying - and expensive if you are using pay per click. Imagine you got 50 clicks at $1 each and none of the clicks led to a sale. You have eaten into your profit by $50 for no return. The sensible thing to do would be to stop the PPC campaign for the product, but how can you if you don't know where your sales came from?

Also - if you are promoting via e-mail marketing you need to know whether or not your list is responding to the product promotion. If they aren't then you are likely annoying them, leading to unsubscribes, complaints and lost opportunities.

So how do we go about tracking? We use TID (Tracking ID's)...