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.
John