The sales funnel is how you handle lead quality.
When you advertise you are going to get a million lookers. Some will take some further action (looking at multiple pages, trying the product , subscribing to an email, etc.)
Every step they take tells you their interest level. Toward the end of your pipeline is where you close them. That's not to say you can't trial close them but real effort goes towards the most likely candidates.
Let me ask, when you advertise are you advertising the product (e.g. Trying to close the sale) or are you developing leads (Advertising a white paper or a newsletter)