Never used a mac, but I assume that is only for installing from the store, correct? If I publish an open source program that one installs with `brew install` or whatever it's called none of this applies?
traditionally you would need to pay the 99 bucks to install a binary on your ipad or iphone or iwatch. those need to have the certificate signed by apple to run on the phone for general security reasons so some website couldn't sideload an app. now you can do all of that without paying the 99 bucks... but if you wanted to sell it on the app store or distribute it for external testing you still need to pay.
Cash cow reasons*. The system is designed to be secure even against signed apps.