Great question mohammad.
a. You tell React how the carousel should look like (using the render function you’re learning just now).
b. You tell React to change the slide when user interacts with the browser.
That’s all. You don’t bother with moving elements in the browser yourself. As the data (such as current slide number) changes, React updates the stuff on the screen itself.
Learning render() is useful because it takes care of the 2.a. part and you wouldn’t be able to use React without it.