What's the purpose of "Instant Dynamic Feature Module"

[chuyun923]

As the documentation says:

• Additionally, when downloading and installing your app, instant-enabled dynamic features are automatically downloaded and installed with your app’s base APK

• "This behavior means that when you set dist:instant="true", you cannot also include <dist:on-demand />."

Then if we can't deliver [dynamic feature] on demand with "Instant App",Why don't we just use the " android library module" instead of "Instant Dynamic Feature Module"?