Once you've got your funnels in place, you have to be able to measure how well the funnel is working; a large part of this is using analytics. Analytics provides the data where you base your measurements on; it can measure many different aspects of how a visitor travels through your funnel and through your website.
This data can prove hugely useful to understand what is working and what isn't and allows you to pivot and make changes that you can then directly track using analytics. Cool, huh?