[acak]

I'm not sure why you were downvoted but it's possible that your question was too basic from the point of view of a web developer.

To answer your question: one reason is that the developer needs to express how elements on a page place themselves not just on the screen but with respect to other elements as well. As you can guess, calculating this "layout" will occur pretty often, and if you have only one thread running all your code, then your logic and communication with the server can interfere with the rendering of the UI - thus making the response choppy.

On native apps, though some of them have borrowed web-type expressions of placement or layout, the entire calculation runs on a dedicated thread. Not on the same thread that also needs to respond to touch / slide / drag events!

HTH.