This is one of the best flattening and Whetstone maintenance products out.
As whetstones are used they become wrapped and uneven. Once they are used enough they can become useless if not flattened. (especially important for single edged knives) This is where lapping plates are come in!
The problem is: most cheaper stone lapping plates can wear just like the stones themselves can. Lapping plates can also become wrapped and useless if not flattened.
Unlike other flattening stones this one will not become even over time.