Board Thread:Speculation, Theories, and Rumors/@comment-30806525-20170206162923/@comment-477304-20170207092502

I don't think it's ever clarified what the Chaos Emerald inside Metal Sonic was for. I think the writers just wanted him to be a literal deus ex machina.

But yeah, I see no difference in a machine controlled my Eggman using Chaos Emeralds and robots doing the same. There's also multiple examples of this: Mecha Sonic, Emerl, Phi, Gemerl, Chaos Gamma, and the multiplayer Shadow Androids (although their canonicity is questionable).