As per Selenium Documents, You may, or may not, want the selenium Server, counting on how you plan to use Selenium-WebDriver. If you'll be solely using the WebDriver API you do not want the Selenium-Server.
If your browser and tests will all run on the same machine, and your tests solely use the WebDriver API, then you do not ought to run the Selenium-Server;
WebDriver will run the browser directly.There are some reasons tho' to use the Selenium-Server with Selenium-WebDriver.You are using Selenium-Grid to scatter your tests over various machines or virtual machines (VMs).
You want to connect to a remote machine that incorporates a specific browser version that's not on your current machine.You are not using the Java bindings (i.e.Python, C#, or Ruby) and would love to use HtmlUnit Driver.