Which is what GPL guarantees, binaries which is guaranteed to provide source code.
Which is why developers who wants to guarantee this right to end users for the code they release, choose to licence their code under GPL.
Only copyleft style licences guarantees this right to end users, so if you are a developer who wants your users to have this right secured (which has a practical benefit for developers as they as end users of modifications to their original code, are guaranteed the source code of those moddifications), you will use copyleft style licencing.
Which is why developers who wants to guarantee this right to end users for the code they release, choose to licence their code under GPL.
Only copyleft style licences guarantees this right to end users, so if you are a developer who wants your users to have this right secured (which has a practical benefit for developers as they as end users of modifications to their original code, are guaranteed the source code of those moddifications), you will use copyleft style licencing.