In today's crazy world, having marketing funnels set up to bring new leads in every week is not optional; I myself have learned this in the last year and am pleased to say that I am about two weeks out from having my own funnel up and running! Why? Because they work.
Honestly, that's the easiest answer I can give. Metric after metric testify to the consistency of results that funnels provide. What are they? Watch for a top level overview.