You need Chrome Driver because it implements WebDriver protocol and communicates with actual Chrome browser.
That way Chrome Driver and Chrome can be developed and maintained independently and Chrome browser code is not cluttered with/impacted by WebDriver protocol implementation.
From https://sites.google.com/a/chromium.org/chromedriver/
WebDriver is an open source tool for automated testing of webapps across many browsers.
It provides capabilities for navigating to web pages, user input, JavaScript execution, and more.
ChromeDriver is a standalone server that implements the W3C WebDriver standard.