It Does A Better Job Than a Mixer
When you mash potatoes with an electric mixer, you run the risk of going too long and the potato starch gets gluey and sticky.
It's not the best texture for mashed potatoes. I like the old fashioned masher. With very little elbow grease, you can mash your spuds plenty smooth, or if you prefer, as I do, you can leave in a few lumps. This tool is also perfect for making skin-on smashed potatoes. You can't do that in a mixer. I recommend the OXO Good Grips Wire Potato Masher.