Every android app has different options to store data as defined here Storage Options | Android Developers
You can decide to install your app to internal or external storage (such as SD Card) as defined here App Install Location | Android Developers
/data directory or Internal Storage is provided to every app as a storage option for any type of files that your app might need but is not (and can't be) the install location of your app. Your app, once installed, is immutable.
I hope this helps!