Mass moment of inertia is a scalar value which tells us how difficult it is to change the rotational velocity of the object around a given rotational axis. When a mass moves further from the axis of rotation it becomes increasingly more difficult to change the rotational velocity of the system. Intuitively, this is because the mass is now carrying more momentum with it around the circle (due to the higher speed) and because the momentum vector is changing more quickly. Both of these effects depend on the distance from the axis. If the assembly represented by the point mass has a small volume or concentrated mass, you may not have to input mass moment of inertia for the point mass because Ixx,Iyy and Izz are small enough to be negligible. If the assembly represented by the point mass has a widely distributed mass, then moment of inertia should be included because it may significantly affect the results. Because mass moment of inertia is part of mass matrix, higher mass moment of inertia generally leads to lower natural frequency.