The Baal Shem Tov explained the mitzva of oneg Shabbat (Shabbat delight) with the following analogy:
A prince who was in captivity amongst coarse people received a letter one day from his father, the king. The prince was delighted and wanted to celebrate, yet he knew that his neighbors could not appreciate the greatness of the king. He therefore distributed liquor and they celebrated, so that he too could celebrate his happy occasion.
Similarly, in order for the soul to rejoice in the spiritual revelations of Shabbat, the body must rejoice with some physical delight.
The Alter Rebbe explains it as follows: On Shabbat, Divine pleasure is revealed, similar to the revelation of Gan Eden. However, since we are physical beings, we cannot experience this spiritual pleasure in its pure form, so it is vested in the pleasure of eating physical food. And in that form we can experience it.
(Maamarim Admur HaZaken HaKitzarim, page 59)
