Newton's laws are not valid if one is working from a noninertial frame. If the frame of reference rotates at a constant angular velocity ω with respect to an inertial frame, we use the pseudo force centrifugal force.
Centrifugal force is assumed act because we describe the particle from a rotating frame which is noninertial and still use Newton's laws.
Example: An observer sitting in a rotating cabin observed that a box is at rest. If the box is at rest, the resultant force on the box has to be zero. The forces on the box are, its weight and the normal contact force between the cabin floor and the box, and the friction force beween the cabin floor and the box. The friction force f = mω²r acts on the box towards the origin. As the weight and the normal contact force cancel each other, to make the resultant zero, a pseudo force with magnitude mω²r is to be assumed which acts on the box away from the centre. This is centrifugal force.