What is terminal used for?
When you work in your OS, no matter what it is; GNU/Linux, Windows or any...
When you work in your OS, no matter what it is; GNU/Linux, Windows or any other, and you use any Gui (graphic user interface) tool like the menu or anything else you are actually using some sort of script that enters a cli (command line interface) command or commands to do what you want done.
The terminal is used to give the cli command directly to your OS.
The gui scripts can only do so much without becoming huge and slow so many times some of the functionality of the cli command is actually not used.
The use of the cli command in terminal is, therefore more effective many times. It is also, most of the time faster.
I mentioned the menu as an example for this reason. You can open any application from your terminal. This is handy when you have a problem opening or using any application. There will be output in the terminal about everything that is done to open and use the application. This may not make a lot of sense to you but will to someone trying to give you help on a mailing list or forum.
In GNU/Linux there are also many very handy and powerful tools for doing an amazing amount of things in OS management like clean up of the file system to sorting files that are not available in a graphic form.
It is not actually neccessary to be fluent in terminal use and many users do not use it ever. It will most likely be something that learning about will be found to be enjoyable and increase, greatly, you knowledge of your OS and how it runs.
On the other hand you can actually do just about anything you can do in a graphical desktop completely in a cli environment.
A fuller answer would need to be based on knowledge of what OS you are running because a lot of tools are in the package management system which is different in different branches of GNU/Linux.
Basic functions like partitioning, listing hardware, files and some diagnostic tools are all common across all distros.
nano, for instance is a terminal based text editor. This is on all distros.