Selenium-RC is the solution for tests that need a little more than just simple browser actions and a linear execution. Selenium-RC leverages the full power of programming languages, creating tests that can do things like read and write external files, make queries to a database, send emails with test reports, and practically anything else a user can do with a normal application.
You will want to use Selenium-RC whenever your test requires logic not supported by running a script from Selenium-IDE
You will want to use Selenium-RC whenever your test requires logic not supported by running a script from Selenium-IDE