As per my understanding of your query, you mean to ask the need for a web driver.
If that's true you should know that Actually, we are implementing "Web driver" which is an interface in selenium.
All browser-dependent drivers like ChromeDriver, FirefoxDriver, InternetExplorerDriver are Java classes that implement the WebDriver interface.
This information is important because if you want to run your program against a different browser you do not need to change a bunch of your code for it to work, you just need to swap out the WebDriver for whichever browser you want.
Hope this helps!