To enable privacy-sensitive users to use the app without sending data to AWS. Only if a user later decides to use backup/sync data across devices he or she can opt-in.
Will AppSync also enable offline-first mobile apps?
For example enable users to use the app without the need to transfer data to AWS (create/modify data without an internet connection on the first app start). And only connect to AWS if the user wants to sign in to backup/sync the app data with AWS.