I think what completes the puzzle is that the Pixel 2 phone omits a headphone jack. Without a headphone jack wireless earbuds are required. Normally wireless earbuds would be superfluous, so they're introduced with a use case that highlights their advantages.

That's to say, I don't believe the Pixel Buds are necessary at all for real time translation. But I think they're presented with the real time translation to give people some positive and cool imagery to associate with the earbuds. It's advertising as far as I can tell.