Why Normal modes of vibration are important when compared to translational and rotation modes. And why the frequencies for rotational and translational modes are close to zero?
Rotational frequencies are not close to zero. They are only smaller than vibration frequencies. You may know that frequencies and energies are proportional at the atomic scale. It does not require much energy to make a molecule move or rotate. More energy is needed if you want to deform it and try to stretch its bonds.