Modules are independant, they add funcionality to the application, but the application should not depend on them, say a users module, adds the posibility to manage users, login, etc. but the application could run wihtout it.
When you preload something it gets (always) automatically imported when creating the application.
But you can use import, when you need a class that will be only used in certain cases:
Say you have a class that inherits from another,
class A extends B{}
if B is not preimported then you need to import it before use it.