This is a frequently asked question, so here is the answer!
Each Expo SDK Version corresponds to a React Native release. For example, SDK 35 corresponds to React Native 0.59. Often there is no difference between the fork for a given a SDK version and its corresponding React Native version, but occasionally we will find issues or want to include some code that hasn’t yet been merged into the release and we will put it in our fork. Using a fork also makes it easier for people to verify that they are using the correct version of React Native for the Expo SDK version – you know that if your SDK Version is set to 35.0.0 then you should use https://github.com/expo/react-native/archive/sdk-35.0.0.tar.gz.