Why Do the Interns Do That?

We have interns at my office. New one's every semester. And each semester/summer the female interns brings cookies towards the end of their time here. It's now happened four times. Why do they do that?

It always makes me feel a little bad, a little guilty, like we should be giving them gifts or something. Why do they bring us cookies? They are coming in, working for free, we should be making them cookies. They don't need to do that. The guy interns are also sometimes similarly generous, leaving food, etc. Before I was here, apparently one guy intern decided to make smoothies and made these disgusting blended strawberry drink no one would drink, except the other assistant who gulped it down to be polite.

I don't get why they all do this.