.Net Report Builder has many uses. It is primarily for programmers or software vendors who want to add Ad-Hoc Report Generating capabilities within their Software Application.
If your clients keep requesting new Reports, and your programmers have to code and create it for them, .Net Report Builder can save your Company time and money by allowing Customers to generate most reports on their own. If you use SSRS (SQL Server Reporting) or other services that charge by CPU cores, you can avoid expensive licensing fees.
.Net Report Builder can also be used for in house Database or a single Application for the same purpose, to easily create Reports.