When to use Selenium server with Selenium WebDriver?

There are some scenario where we have to use the Selenium-Server with Selenium-WebDriver.

1. If we are using Selenium-Grid to distribute your tests over multiple machines or virtual machines (VMs).
2. If we want to connect to a remote machine that has a particular browser version that is not on our current machine.
3. If we are not using the Java bindings (i.e. Python, C#, or Ruby) and would like to use HTMLunit driver