Ah, etiquette.
Back in the middle ages, nobles would give parties for other nobles to attend, but since they didn't have a way to check if everyone inside was a noble and/or someone related, there were many uninvited guests among them. In order to filter out the uninvited ones, they decided to create a secret code called rules of etiquette, a set of behaviors they would abide by to prove they were properly invited, and those not adhering to the code would be kicked out.
Unfortunately, as time passed on, people started attributing etiquette with "good manners" and incorporating it aimlessly in their culture. I grew up hearing all the time putting your elbows on the table is bad manners and never understood why until I learned about etiquette. Turns out it's not bad manners, it is against the code of etiquette.
This is why it's important to learn the origin of the customs you practice, otherwise you're just doing stupid thing without even realizing you're looking like an idiot.
