Analytics is a fantastic tool because it eliminates the guesswork as to how visitors behave on a website. Conversion rates are one of the top priorities for many marketing websites, and Analytics can certainly help give you insight as to why the conversion rate is what it is. It can also shed light on additional user experience issues.
Google Analytics is a tool for improving products, not just marketing websites. It tells you exactly what users are doing, where they’re coming from, how long they engaged, and so on.