Uncle Simon - S5-E8
Trivia: Uncle Simon's mechanical man is a thinly-disguised Robby the Robot from Forbidden Planet, with a different head and "face" under his glass dome. Robby guest-stars with his original head a few Twilight Zone episodes later, in "The Brain Center at Whipple's." (00:17:00)
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.