You might not want to eat 미역국(seaweed soup?)right before some important test, interview or exam, whatever. Since the seaweed is very slippery, it's considered unlucky.
In Korean, they use 붙다 to stick for an exam.
ex) 시험에 붙다/합격하다
So, slippery things are not good for sticking to something.
And Korean's reply "미역국 먹었다", when you ask if one passes the test, means one failed to pass. Similar expression, "미끄러졌다." is also unsuccessful.
So Koreans eat glutinous rice cake (찹쌀떡) or taffy (엿) for the wish of successful result.
And stick to some group is crucial key to be a member of any organization, you can't say it's just superstition. It could mean their commtment to the group they belong to. If you're slippery member who can move to any where, any time, Korean won't believe you as a true member.