Why You Need Behavioral Targeting In E-Commerce Search
Site search engines in the e-commerce world have a tendency to operate in isolation. They receive a query and return results. This works well till the site reaches a couple of thresholds. These are: 
when you cannot improve the accuracy or quality of search results anymore through rule changes.

when the human effort required to manage promotions and merchandizing is enormous.

when the buzz around certain products is so high and variable from day to day and week to week, that by the time business rules change, the popularity has ebbed or the pattern has changed.