It is tradition!

In every single company I have been at, everybody gives year end "appreciation" cash to assistants, and every assistant receives cash from the more senior folks they cover. It is meant to show personal appreciation, so it is out of everyone's personal pocket. at my current work place, there are five of us who share one assistant. We together gave her a Xmas card with $3,000 of cash last week. The amount would have been higher had it been a better year for the company and had all of us got paid a bit better.

It is what you deserve. Just take it. don't think too much.