If you look at the overall behavior of all of your website visitors, you'll be inundated with a ton of information. Trying to distinguish and identify patterns among such a large group is extremely difficult, if not impossible.
If you segment or divide your users into logical groups and then compare and contrast how they behave and what they do, you'll have more accurate, actionable information that actually means something. Segmenting your website visitors allows you to develop a clearer overall picture of your site's performance. If the metric you're trying to measure is how many people buy a particular product, it makes sense to measure that within the context of how many people interacted with the site in a meaningful way.
Since prospects and customers are always more likely to respond to messages that match their interest sets, it makes sense to chop the database into smaller, segmented slices based on preferences and past behaviors, making email messages as relevant as possible to each group. You may use historical data to apply what you know about the likes and dislikes of your target market segments, and develop your messages and offers to fit.