Discussion
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.
My very non-canonical head canon of the Star Wars galaxy is that it's the Milky Way in the distant future. At some point, humans, with their droids and hyperspace, colonized the galaxy entirely. This is where the widespread tech and omnipresence of humans comes from. At some point, this civilization was overthrown by nonhumans rebelling en masse, a rebellion which includes droids.
It's from this collapse that the Unknown Regions become unknown.
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The implied model of work, here, is all sorts of special purpose droid labor units -- think factory construction bots, farming bots, etc. -- managed by a smaller number of protocol droids who can speak to them. Likewise, the protocol droids are also employed to speak to nonhumans in their own languages, so the humans can talk to and, in standard colonial trope, give _them_ instructions as well.