1.    It justifies the filling of ns orbital in preference to (n-1) d-orbital. For example, K has an electronic configuration:
                                                1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹
Here Z_eff  for for 4s = 19-[(10 x 1.00) + (8 x 0.85)] = 2.20
If the electronic configuration were

Here Z_eff for 3d = 19 - [(18x 1.00)+(0.35)] - 1.00.
This shows that a 4s electron experiences a z_eff and hence a configuration in which 4s is occupied in preference to 3d would be more stable.

2.    Although stator rules do not allow the exact calculations of the energies of electrons yet these are useful in understanding variation in size, ionization potential and electro negativity.
For example , for an outer electron in elements along the 3rd period, we notice the following tend in Z_eff  values :
Element                   P                  S                 CI                Ar                       K
Z_eff                            4.80           5.45           6.10          6.75                  (2.2)
This shows that Zeff increases along the period and reaches a maximum at the noble gas which justifies increasing ionization potential along the period with peaking maximum at the noble gas. Ionization potential shows a sudden drop after that as evidenced in  Z_eff  value of K.

3. It helps to justify the removal of outer ns-electrons before the removal of inner (n-1) d electrons. For example, it can explain why an outer 5s electronics removed before 4d electron in the case of Ag (Z = 47, 1s2 2s2 2p6 3s2 3p6 3d10, 4s2 4p6 4d10, 5s1). As given in example the effective nuclear change Z_eff  for 5s electron is 3.70 and for 4d electron it is 7.85 . This means the 4d electron is held much more strongly by the nucleus (Z_{eff =} 7.85) than the 5s electron (Z_{eff  = 3.70)}. This means in the ionization of Ag atom , the electron ionize first form 5s-orbital because these are least tightly held.

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