In a universe brimming with untold sentient species, all chattering away in their own languages and displaying specific customs and behaviors, go-betweens were critical. That's where protocol droids like C-3PO came in. They spoke millions of languages and translated culture, too, smoothing interactions between species that might otherwise attack each other with reckless abandon.
The RA-7 was one of many protocol droids, with purposely designed blandness to help it stroll through everyday life without much notice. The designs is similar to that of C-3PO, but with a slightly more insectoid head.
The droids' banal appearance (and aloof personalities) belied a sinister purpose. Their circuitry included covert surveillance technologies that allowed them to spy on their surroundings, making them especially useful to the nefarious scheming of Imperial officers of the Galactic Empire.
There were scads of these things aboard the Death Star, where they reported any suspicious happenings to their evil handlers, leading to the arrest and disappearance of thousands. Most RA-7s perished along with the Death Star.