Hey Numan, Selenium Server is not required by Selenium WebDriver when we run the Selenium Automation tests on the same machine i.e. Browsers and Automation tests are on the same machine. Selenium WebDriver makes direct calls to the browser using each browser's native support for Test Automation.
But, you may require Selenium Server for running Selenium WebDriver Automation tests on different machines i.e. when the Selenium tests and Test Browsers are on different machines. This may be possible while implementing Selenium Grid where you have to run automation tests simultaneously on different machines to reduce time.