Many translators aren't comfortable with editing source code. They want to get a file, e.g. in XLIFF format [1], which they can then load into their translation management system, and then use a GUI or web based tool to enter the translations for each string, and then at the end export the updated file and send it back to the developers. Plus, the translation management system makes it easy to keep track of progress (what % of strings have been translated so far), automate review (require each translation to be signed off by a second translator), etc – but, translation management systems can't import/export source code, but they often can import/export message catalogs.

[1] https://en.wikipedia.org/wiki/XLIFF