Surface Area must be maximized for cells and all over the body. This allows for maximum productivity with a low use of energy.

Basic Surface Area to Volume Ratio is 6:1

Examples:

Why are there so many little sacs as opposed to one big sac.

Alveoli in lungs:

What happens on the inner membrane? What is the reason for the cristae?

Mitochondria cristae:

Why are there so many curves in a thin tube as opposed to a big sac like the stomach?

Small Intestine and Large Intestine interior: