It is generally acknowledged by computer pros that you can increase your productivity and accomplish more tasks by using the keyboard instead of mouse. In other words, its easier and gets your job done faster:). Take for example the task of selecting all the text in a document. You can do that by simply pressing Ctrl+A while doing that with the mouse requires pressing the left mouse button and dragging it across the whole document. In time you will learn that keyboard shortcuts let you do more with less effort.
Derives from the first motive and basically means that when you use the keyboard you dont have to follow a pointer to see what you are doing and you can do that thing semi-automatically thus freeing your mind for other tasks.
You can combine the advantages of using the keyboard with those of using the mouse. There are tasks that are much easier done with the mouse, take the example of browsing the web pages and clicking links but at the same time you can use the keyboard to navigate within the page or between tabs. And the beauty of it is that you can do both at the same time.
Extensive mouse usage is associated with R.S.I. (Repetitive Syndrome Injury) much often than the keyboard usage. By alternating mouse and keyboard usage you can reduce the risks of R.S.I. and reduce the fatigue your wrists and fingers.
When you have a job that requires precision it is advisable that you use the keyboard, for example if you do a lot of text editing, it is more accurate to handle it through shortcut keys.