The aorta supplies the blood that is being filtered by the kidneys.
The urinary system is the system that filters the blood, removing waste and excess water, and eliminates them through the creation of urine. The kidneys do the filtering and then urine is sent to the bladder where, when there’s enough, we get the urge to pee.
Because the blood is being filtered by the kidneys, it’s important that they be located somewhere that a lot of blood can filter through, and that a sufficiently big artery run through it. This is why the kidneys are located just below the ribs in the upper abdomen and that a major branch of the aorta runs through.
The entire volume of blood runs through the kidneys 300 times per day!