"Playing devil's advocate" means to argue or present the opinions of the opposite side even though the person doesn't agree with the opinion they are presenting. In order for someone to be playing devil's advocate, they must be arguing or presenting a position that is the opposite of what they actually believe.
The purpose is to highlight what the opposing side feels and hopefully give another person some insight into why there is an opposition and how that opposition might react. Playing Devil's advocate is most commonly used when helping to make a decision.
It's very important to understand the difference between arguing an opinion and playing devil's advocate. If a person genuinely disagrees with an opinion and argues the opposite, they are not playing devil's advocate. They are stating their opinion. It is only a devil's advocate when the person is arguing or presenting an opinion that is the opposite of what they believe.
Playing devil's advocate can be a very helpful thing when done for the right reasons. By presenting and arguing a counter opinion, it can help people to see a different perspective and educate them on how other people think. It can be very helpful in preparing for a debate, understanding a sales market, or getting someone prepared to make a decision.
Historic background:
At one time the Roman Catholic Church had an official position that was termed the devil's advocate. The job entailed finding and presenting negative information on a person who passed away and was being considered for canonization. The idea was to present all the positive things the person did with negative as well.
- https://study.com/learn/lesson/devils-advocate.html# with editing
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Growing up with 稻香村 cookies, I always love eating them. For the past three years, Wlamart here has been selling them around Mid-Autumn Festival and Chinese New Year, and I bought close to 20 boxes (牛舌饼，桃酥) last year alone.
But for whatever reason, they are missing this year. Instead, the store is selling some fancy looking mooncakes. I bought one box last week (pic #1), only to find that the white portion around the egg yolk ain't egg white, but lard fat (pic #2)! 
Hubby has a sweet tooth, so he played devil's advocate and asked me not to throw the mooncakes away before trying.
So I tried some after taking the fat out (pic #3), and found it's still too rich for my taste. 
I'm sure 白云 can make much better mooncakes, maybe even with some tomatoes. 
Cheers!
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#3
