As someone, who has not used Ninja, what advantage is there, compared to Makefiles? And is it worth introducing yet another tool, to translate one to the other? Especially, when the Ninja files are that huge, possibly human-unreadable.

The Ninja files being that huge is likely more to do with the Android build environment or the tool that generates them. The main advantages of Ninja as a build executor are that the language is simple and it processes the build graph very quickly.

They are huge because android has hundreds of smallish makefiles but the generated ninja file is a single flat file.

The advantage in android is that the different build systems will generate ninja, so they can interoperate.