I have read some tutorials about creating an API and i wonder if XML-RPC and XML-RPC server in CodeIgniter are for creating this?

If yes, what are the benefits of using these classes rather than im just using regular class/methods/params to create my API interface.

This is what it said in the documentation:

Quite simply it is a way for two computers to communicate over the internet using XML. One computer, which we will call the client, sends an XML-RPC request to another computer, which we will call the server. Once the server receives and processes the request it will send back a response to the client.