When we do denormalization in our database design

From BDOTNET forum,

The main reason for renormalization is leverage performance and helpful for ad-hoc reporting . In some situation a database design in 3rd normal form may require more table joins to process a query this additional table joins can degrade performance , rather than it can be achieved in 2nd or 1st normal form.

Another reason to denormalize a database is to simplify ad-hoc reporting. Ad-hoc reporting is the unstructured reporting and querying performed by end users. End users are often confused when they have to join a significant number of tables. To avoid the confusion, you can create a special set of tables designed for ad-hoc reporting.