Hashem tells the Jews that he knows they will eventually sin and He warns them not to. In 23:25 Hashem concludes, "If you serve Hashem, he will bless your bread and your water and remove sickness from amongst you."
Why is that the reward for following Hashem faithfully? Why bread and water? Why not say I will give you wealth, or whatever. Why bread and water?
Bread and water are the basic staples needed to survive. The Jews already learned from the incident by Mara that wealth and other promises do not matter, if the basics such as bread and water are not covered.
That is why Hashem promises them if they serve Him, He will ensure their health and their supply of bread and water. Anything beyond that is extra, but these are the basic needs.