As far as I know, a drawing tablet is basically a computer peripheral that acts as a substitute for the mouse. As you can use a pen to draw digitally, it makes the task easier for people who are not used to or uncomfortable with using a mouse to draw.
It should be obvious, atleast it is to me, that drawing a picture using a pen will make the action more instinctive, as we use the pen much more compared to the mouse.
I think it's different for people who are comfortable drawing with a mouse, though.
Edit - It can also be a separate computer altogether, like HP Touchsmart tm2t. Some people choose tablets like that just because of the graphics applications.
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