Office Christmas parties are ostensibly about celebrating a year well done or a reward for twelve months of hard work, but they're also an opportunity to flex. They're for showing the people you see everyday what your life is outside of work, to the extent that you're allowed to have one.
A guy took his office Christmas party so incredibly seriously that it caused a rift at home. He posted on "Am I The A**hole?":
This happened a few weeks ago but my (30M) wife (30F) is still upset about it, so I am bringing this to Reddit. My company had a Christmas party a few weeks ago, and employees were allowed to bring their significant others. It was a fairly fancy event and everyone was expected to dress up. When my wife came downstairs ready to get in the car to go to the restaurant where the event was being held, she was wearing appropriate clothing for a fancy dinner, but she was wearing her glasses.