Mr. Dingle, the Strong - S2-E19
Factual error: Though this episode is set in then-present-day 1961, the fistful of prop money the bartender is holding is not contemporary. It appears to be copies of 1880s U.S. Currency, though it's been reproduced as smaller, 20th Century sized bills rather than the larger ones that were standard in the 1800s. All in all, not something his bar patrons would likely be paying the bartender with in the 1960s. (00:16:40)
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.