3P0 -- and other Protocol Droids -- are there to translate non-human laboring droids into human, and to convey human instructions to the laboring droids. Of course, the laboring droids *understand* human languages, they just aren't built to produce them. It's important they can do what they're told, moreso than for them to give information to Humans.
The fact that cyborgs are apparently in this category as well tells us something about bio-essentialism in the Galaxy, too.