Why we require a separate browser executable file to launch a browser in selenium?

It is very frequently asked interview question for me. I was no answer as till now we know only we need drivers( like chromedriver.exe, geckodriver.exe, IEDriverServer.exe) and pass them to the System.setProperty() to launch a browser using selenium web driver. But why?
Reason:
As selenium WebDriver has no native implementation of a browser, we have to direct all the webdriver commands through a driver server (driver file).
Driver server is a standalone server which implements WebDriver's wire protocol.
Wire protocol defines a RESTful web service using JSON over HTTP and is implemented in request/response pairs of "commands" and "responses".
The wire protocol will inherit its status codes from those used by the Driver Server.
Each Driver implements Webdriver protocol and starts a server on your system.  All your tests communicate to this server to run your tests
Note: Each browser contains web browser engine. To display the web content, the web browser engine is required. There are different web browser engines and driver servers for each web browsers.


Hope this helps!!!