On the Apple guide, there is a step before building the AppStore version:
Open the Xcode project and Duplicate the “Release” configuration in the Configurations pane of the project's Info panel. Rename this new configuration “Distribution”.
Why this step is needed? Can I skip this step and use the "Release" configuration to build the final version for AppStore?
You could name it "DebbieGibson" as long as you remember you set that configuration up for App Store distribution.
If you keep your release and appstore configurations separate you're able then to test your application built with exactly the same settings as it will go to appstore and switch to that configuration with 1 click. It may be not necessary but (imo) is really handy.
Another reason may be that while developing you may want to play a bit with some project settings and be sure that your experiments won't go to distribution build by any chance.