Eating on Shabbos is Spiritual Sustenance

Sefer Zera Shimshon (Parshas Eikev) writes that although we eat Mazon Gufni (physical food) on Shabbos, it is considered to be “Mazon Ruchni" (spiritual food). This is as the Reishis Chohchmah (Shaar Hakedusha, Perek 15, Ois 77) writes that whatever meat and fat is added to a person's body as a result of the food he eats on Shabbos will never rot away or be eaten by worms after his death.

This fits wells with Chazal's statement (Pesachim 100B): “Ein Kiddush ela b'makom seudah." (Kiddush can only be in the place of a meal.) These words can be understood to mean that since the Shabbos seudah is a holy, spiritual meal, one must first make Kiddush to symbolize that this repast is completely sanctified.