A, Rapid killing of TB bacilli, to produce rapid improvement of the clinical condition of the patients
B, Prevent the emergency or worsening of drugs... resistance
C, Prevent the relapse of disease after the completion of therapy and achieve long lasting cure more

Because it takes along time for drugs to reach and be incontact with the tuberculous bacteria and malnutrition

To prevent relapse of tubercule baccili

To ensure the latent form of the mycobacterium is eradicated fully to prevent relapse.