A terminal emulator gives you access to a command line interface. The command line interface is a "view" of your operating system. Using a command line interface, you can look create, read, update and delete files. You can run programs, and you can manage and view important parts of your operating system.
Please do read this wiki article about terminal emulators.
So the answer to your question is the role of terminal allows us to use our computer via a text-based interface.