What is the Function of Thanksgiving?

In one of my favorite passages from his writings, Peter Achinstein distinguishes between three meanings of the word "function,"

Suppose that a magnificent chair was designed as a throne for a king, i.e., it was designed to seat the king. However, it is actually used by the king's guards to block a doorway in the palace. Finally, suppose that although the guards attempt to block the doorway by means of that chair they are unsuccessful. The chair is so beautiful that it draws crowds to the palace to view it, and people walk through the doorway all around the chair to gaze at it. But its drawing such crowds does have the beneficial effect of inducing more financial contributions for the upkeep of the palace, although this was not something intended. What is the function of the chair?

Achinstein argues that the question is ambiguous and we need to look at three things. The design-function of the chair is to seat the king -- that's what it was designed to accomplish. The use-function is to block the doorway -- that is what it is intentionally used for. The service-function is to raise money for the upkeep of the palace -- that's what it actually does.

I want to ask the same question about Thanksgiving. Now, the design-function of Thanksgiving is tied up in the mythology of the supposed first feast with the Pilgrims and the Indians, it involves bringing people together and being thankful for the bounty we have received from the Earth. The use-function, I argued a few weeks back is to provide a safe social space for gluttony, a chance to indulge and not feel guilty. But what is the service-function? What do we actually get from Thanksgiving? In the end, what is the sociological pay-off?

Surely, family togetherness is a part of it. The regular high school football rivalry games enlarge the scope to reveal community togetherness. The long weekend including Black Friday helps the holiday serve a financial function. Others?

What is the service-function of Thanksgiving?