It isn’t necessary but it does kill any bacteria on the shells. And yes it does make them easier to crush. As far as the shells alone not being enough calcium , I have yet to see that. We’ve been using shells alone for almost a year now and have had no problems. Everyone is entitled to their own...
We feed our girls their egg shells for calcium. It works great and is very cost effective. Also if you bake them it adds a different taste and texture that they enjoy.