Explanation below. Also, there is a similar thread on the same topic here: https://www.edureka.co/community/815/selenium-server-standalone-selenium-webdriver-different
In Selenium RC (Remote Control), it was mandatory to use selenium-server-standalone.jar jar file because RC involves a server and RC Core will then use this server to establish a communication between the browser and your code. All this is powered by the selenium-server-standalone.jar. along with library functions present in your code.
But with Selenium WebDriver, there is no need for selenium-server-standalone.jar jar file, because the WebDriver API directly communicates with the browser's native language and overcomes the Same Origin Policy issue. So, the selenium-java.jar jar files is used instead for testing with WebDriver.
Hope the answer helps.