As a user, you get an application that uses the library that application X used to export its files.

Or you get a version of application X compiled for your new platform, that can save to an intermediate format loadable by application Y.

Or you get application Y with an implementation of the file format for application X because the developers were free to study application X's code.

Those are the benefits you get as a user. Better code, usable in more scenarios, that better supports your freedom to use it.

The GPL ensures that freedom.