There is not so much to write about AsyncStorage, the fact that it's used instead of LocalStorage on the web. You save authentication data or any other small data, you may probably need later on.
It's important to understand that AsyncStorage is unencrypted, so if you want to store passwords, make sure that you encrypt them before that with packages like crypto-js or bcrypt. Ideally your password should be encrypted on server.
Although it's possible to save data in LocalStorage, it's discouraged and advised to use AsyncStorage instead.
On the iOS side, AsyncStorage uses native code and stores key-value pairs in a serialized dictionary, when dealing with short values. Large values, though, are stored in files. On Android, it uses RocksDB or SQLite based on what's available.
AsyncStorage is promise-based,...