As we know there are two kind of Caching is available in OBIEE .One of them is most popular and familiar i.e. BI Server Caching and other is BI Web Server caching . Presentation Catalog Caching is rarely used and it rarely take any attraction in performance . However a most significant use of it will be, bypassing the default report level Caching in real-time BI application i.e typical Transactional Application where the back-end data changed in a moment and need to be reflected in the Analytics report (though BI is not built for OLTP query) .In such scenario it would be meaningful for developers to force bypass the web query caching and let analytics hit the database once again to fetch updated data , if any .This could be manually done by pressing Refresh button at the bottom left of Browser window .
However if there are multiple dashboards and say user1 wants dashboard1 data updated on real-time after back-end ETL populates, then rather suggesting user to click on Refresh button each time is not the best option as fickle minded user1 could see stale data and can raise a hue and cry unnecessarily on old data .That could be easily avoided using Bypass Web Server Cache feature in Advanced tab of the corresponding reports .
This feature would not only giving the user flexibilities to customize the set of reports where the real-time data reflection needed but also avoiding a global change in cache config params under instanceconfig.xml which will be applicable for entire platform and possibly that could not be the right choice at right time !