The short answer is no, you don't need one; however, they do make some jobs not just easier but more enjoyable. Not all tablets are created equal, though, so this photographer gives his take on a brand you might not have heard of before.
In this video, photographer Mark Duffy reviews XP-Pen's Deco Pro graphics tablet. While you might not need one of these, as Duffy states in the video, it's so much easier to dodge and burn in Photoshop with a pen than it is with a mouse. When processing like this in Photoshop, the constant left button clicking with a mouse can end up giving you carpal tunnel, not to mention the fact that it's just plain irritating. A graphics tablet solves all that, and the experience of getting lost in an image while editing makes it much more enjoyable. It's just more fluid and tactile.
Most of you will probably have heard of the Wacom Intuos Pro graphics tablet. It's a favorite among illustrators and designers, as well as photographers. For good reason, too: it's well-built, reliable, and accurate. But all of this comes with a fairly high price tag. It's not bank-breaking, but for something that you might not be using all the time, it's a relatively big investment. So, having options like the one in Duffy's video is a welcome change for those who want to try out using a tablet in their workflow, but who can't really justify a Wacom price tag.