A touch of art deco with a hint of malice. This is the penultimate Droid for me to render and it feels good. The Mk 7 was originally a mobile colony factory affectionately called a "Big Eye" that the Droids re-purposed. You can read up more on it on the blog.
Good question! The answer is in the history. The Droids didn't design themselves, they were designed by humans. When the Droids took over control of the Imperial artificial intelligence, it took advantage of all those designs that it felt were useful to killing all life. The Mk 7 were originally used by humans for building colonies (called Big Eyes). The Droids cannot innovate, so they simply built the designs that were there.