What is the purpose of reabsorption in the nephron?

To retain useful substances in the body such as glucose as well as reabsorb appropriate amounts of salts and water.

Without re-absorption the blood would be too thick and many vital minerals and components would be lost in urine. This re-absorption is of two types:absorption with the concentration gradient and the other is Active re-absorption against the gradient which requires ATP to be spent.When ultrafilteration occurs , there is limited check as to what has to be excreted out of the blood. The  cellular component is retained. But once the filtration is done and the blood flows through the Efferent arteriole the process called selective re-absorption takes place for the said reasons.