I am a student studying portraiture. I do figure drawing in graphite for 10 hours a day. This eraser is essential basic equipment. It works well on cheap sketch paper, and is even better on expensive papers such as Canson and Roma. The eraser lifts off graphite without a lot of pressure, so it doesn't do much damage to the paper surface. Obviously, rubbing hard will do damage, no matter what eraser you use, but this one is soft enough to yield under slight pressure. It leaves crumbs on the paper, which may be blown or brushed away. I use small brush for this purpose, so as to avoid touching the paper or the graphite.
This eraser is also excellent for precision erasing. For example, if you need to create very thin strands for suggesting hair in graphite, or for creating a contrasting edge. There are two ways to use it for precision work. 1. Use a scalpel to cut a clean edge, or better still... 2. Use that cut-off bit to do the precision work. This gives you a thin edge all-round and wastes less of the eraser. For very delicate work though, buy the Staedtler eraser pen instead. It's much more efficient and easier to use in tight spaces, though it still requires cutting sometimes for fine work.
It doesn't generally collect dirt, but it can get a bit grimy after continuous use. If you are working on something important, it's a good idea to test-rub it on a scrap area first, which will also clean the edge for you.
It also erases charcoal residue. If you have erased charcoal but have still got a faint shadow of it on the paper, this eraser makes a good last-resort if you need to shift that shadow. It doesn't always do the trick, but it is usually worth a go.
These erasers last a long time. I got through just two of them in a year of drawing. That said, I buy them in multiple packs as they are cheaper, and I can sell the spare ones to other students.