That's not how ad performance is measured. Your ROI is based on end conversions. You need to know if your leads are _good_, not just plentiful. A company that doesn't do this at the start will figure it out pretty quickly.
Ideally that would be the case that all ad-spend can be tracked through end conversions, and for many businesses it can be. But for much of the corporate world, you don't know where your end conversions come from. (Think Nike shoes, etc)
It's estimated about that 35% to 45% of all digital ad spend goes to brand awareness, video reach, and other upper-funnel methods where direct conversion tracking isn't possible.
