How client side works?

Question

What is the use of it?

Answer 1

If I get your question right, you're asking for the purpose and the way client-side scripts work?

Well, in the beginning everything is on your server, if a browser calls an html-page from your server it copies it and all the scripts referenced. Meaning it caches the javascripts and uses this local copy to execute stuff. Hence it uses the copy, the client doesn't need to run a request to the server for every bit a script should handle, keeping the traffic of the server low and the server responsive to more requests and speeding up the client's performance.
The downside is, the scripts are accessible to the client and, what is worse in terms of security, they can be altered by the client.