Many websites seeks to build a community of visitors. These websites want to meet their community's wants and needs but all too often, we as webmasters are poorly equipped to learn or understand those wants and needs.
If we owned and operated a traditional bricks and mortar store, we would see the expressions on our customers' faces. At a glance, we could see if our customers look confused, content, frustrated, eager, etc... We could ask them if they need help and find out more about what they want. We would even be able to eyeball rudimentary customer demographics just by spending time in the store observing customers.
Unfortunately, we are webmasters and as such we see our visitors as sequences of HTTP requests from a particular IP. Though we can get some important data from our log files, we can't see the looks of frustration or confusion on our visitors' faces. When they can't find something they can't just ask someone next to them for help. You may tell yourself that they can always e-mail you any questions, and some of them will, but the overhead and time delay involved in sending a question via e-mail will cause most unhappy visitors to leave instead.