Combination chemotherapy is used in tuberculosis in order to reduce:?
Combination therapy:
Like in the case of TB, leprosy, HIV and malaria, combination therapy with 2 effective drugs has several advantages in the treatment of VL. These are:

Limiting  risk  of  development  of  drug resistance,  thereby  prolonging  the  effective life-time  of  available  medicines.
Attaining  higher  efficacy  and  cure  rate.
Shortening of duration of therapeutic regimen; better  compliance  and  convenience.
Reduction of overall dose; lower toxicity and cost.

Reference: Essentials of Medical Pharmacology Seventh Edition KD TRIPATHI  page no 847
Side effects of drugs
Cost of therapy
Dosage of drugs
Development of drug resistance