What mechanism is primarily prevented by using fairleads on a line?

Using fairleads on a line primarily prevents chafing. Fairleads are designed to direct the line smoothly and reduce the angle at which it passes through or around an obstacle. When lines run through fairleads, the chances of the line rubbing against sharp edges or rough surfaces decrease significantly. This is crucial because chafing can lead to the weakening and eventual failure of the line due to wear and tear.

By keeping the line properly aligned and protected, fairleads contribute to the longevity and reliability of the line under various loads and conditions. This protective function is particularly important in areas of high friction or where lines might experience significant movement or tension, ensuring that the line maintains its strength and doesn't break down prematurely.