Web traffic reports and other Web analytics tools such as online testing are excellent for finding out what visitors are doing on a Web site and how frequently the actions occur.
When it comes to finding out why Web site visitors are behaving as they are, you have to interact with real people. To understand why, watch what they do, how they behave, and talk to them.
This can be done through field studies (more on uie.com), or using a similar research techique, ethnographic research. In "Offline Research for the Online World", Dave Friedman of Avenue A Razorfish, talks specifically about how ethnographic research can provide information essential to successful design:
"...By understanding how consumers perform a task in the offline or “real” world, you can design more-appropriate online tools or site navigation structures to help consumers perform the same task online in a manner that feels natural to them. It seems like a simple concept, yet many marketers expect to find answers to online problems through analytics—which fails to provide the much-needed behavioral or motivational perspective..."To capture the motivational and behavioural learnings from field studies or ethnographic research, create personas. Then use these personas to construct the scenarios and conversion funnels that you'll analyze using Web analytics tools.