For years I didn't - I had an old Wacom that gathered dust. Then last year I worked the Focus on Imaging show for Adobe and forced myself to use the loaned Intuos 4, mainly to see if I'd changed my views. I ended up buying one, and now use it routinely with LR's local adjustment tool and spotting tool, and with PS for painting on masks - again local adjustment work. But not for much else. So at this year's show 3 of us were discussing tablets - one person hates them, another loves them, and I've wobbled. In other words, you certainly do not need one, and you can only try and see if it suits you.