Why do I need to be numbed for a crown impression?

You didn't give very much information so I am not exactly sure of your situation. Usually when a crown is being made there will be some tooth structure removed. This can be painful without numbing. The impression itself isn't painful, but the rest of the process may be.

In order to take an impression for a crown, a small string or cord needs to be tucked under the gum all around the tooth. For most people this can be uncomfortable and it is important that it be done well to get a good impression.