It was not meant to be convertable back to php files.
Its just as a help to improve translations with the tools that knows PO files.
Find mistakes that would otherwise be hard to find.
The real advantage would be if phpBB switched to using PO files for translations.
Then translators could focus on translating and using the many tools instead of looking at PHP files in text editors.
I dont know how hard to would be switch phpBB to using PO files. Maybe if the code is well written it is not hard even though its a project with alot of code.