Prayer focuses on God as the One who supplies. It acknowledges that He is the source of all our physical needs, and it teaches us to live one day at a time in the confidence that He will meet those needs.
Just enough. Ever wondered how he was able to give them manna for two days when the sabbath was coming up, and yet on any other day he gave them manna for just that day? God draws us into dependence on him. Give us THIS DAY our daily bread.