Question

What is the need of converting sliding friction into rolling friction?

Answer 1

There is a need of converting sliding friction to rolling friction because the friction becomes less and easy to move an object/.....

Answer 2

When the motion of objects is translatory - in a straight line - there is kinetic friction that opposes the motion. When an object is rolling then it is the rolling friction that is applied. Rolling friction is much less compared to the sliding friction on a hard road. 
However, rolling friction also is high on sandy beaches.
Friction does not depend on the area of contact. Mainly depends on the coefficient of  friction, weight, and type of surfaces in contact.