You need to use lapping plate or flattening stones to make sure your stone is flat. If your stone is not flat, it will remove metal from your knife only in areas where stone contacts knife. It is very rare when new purchased stone is flat. Almost all of them come from manufacturers not flat. Even expensive ones.
Another reason to flatten your stone when it is used for some time and it is dished. "Dished" means that stone is thinner in the middle.
If your stone is not flat, you will be damaging profile of your knife and will have to correct it later. It doesn't make sense to use not flat stone.
Note that you don't need to flatten diamond stones.
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