The Shell is a powerful tool of the GNU/Linux operating system. The shell is an interface between the user and the linux kernel. Long before the GUI - graphical user interfaces became common, the shell (variously called "the the command prompt", CLI (command line interface), console prompt) was the only tool by which the user could instruct the computer.

Why Shell? - A quick FAQ

1.Why should I learn to use the shell? Can't I do everything through the GUI?

It is true that the GUI is itself quite powerful, and using the shell requires a fair amount of typing, but once you get used to it, you will find that for many things the shell is far more superior. For example, using regular expressions, one can make complex selections of files, which cannot be done with the mouse.

2.Aarrgh! I hate typing on this keyboard! It is easy to double click rather than typing "cd my_dir/"

You don't really need to type too much on the shell, because most Linux shells will "autocomplete" file/command names. Typing is painful, if you are not used to it/have a horrible keyboard. The best solution is to practice/get a better keyboard.

3.Do I have to mug all these commands?

No, not at all. You can use them when you need them. If you are searching for any command, just google for common shell commands. You'll remember most of the useful commands once you begin using them.