One of the big challenges in search engine marketing is getting all of the volume you want from your campaign.  However, the available volume in paid search is sometimes limited, and prevents you from getting to where you want to in your campaign. In other words, people only search so much on keywords related to your campaign, and there is nothing you can do with your paid search campaign that can change that. Getting at as much (profitable) volume as you can is what it ends up being all about.