Rice symbolizes food, including not only spiritual food, but also food in the usual sense. Besides, "rice" means "money" in many places in China, so sending rice has rich meanings.
In ancient times, rice was the salary (salary) of ancient officials. Because of the turmoil in ancient times and the shortage of food during the war, sending food means sending money to ensure safety. Moreover, this folk custom has spread to this day, and now some merchants are escorting rice between friends when they open their business, hoping that their business will be prosperous and their financial resources will be extensive.