Repertory is an index of homoeopathic materia medica which is full of information collected from toxicology, drug proving and clinical experiences.
The process of repertoriasation is essentially an elimination, which starts with a broad choice and slowly narrows   down the field, giving us adequate small group of medicine, so that the final selection is made easier with the help of further reference to the materia medica.
The physician who have never used or have  rarely used repertory complains about its elaborate methods and time consuming nature, but one who has used it meaningfully find it is highly useful and time saving.

1. To avoid routinism 
For a stomach pain ameliorated by hard pressure a routine prescriber settles his prescription upon Bryonia, but in repertory under Stomach - pain-pressure ameliorate, there are 7 remedies but no Bryonia. Repertory teaches the physician to be careful in the selection of the medicine and avoid routionism.

2.It teaches by gradation the relative importance of various medicines
It is important that symptoms according to their intensity [gradation] in a patient must match with similar intensity in the medicine chosen. An ideal homoeopath make an analysis of each symptom in each case and make the final selection according to the intensity and gradation of each symptom of the patient as found in the medicine.

3.To select similimum quickly.
Repertory reduces the laborious work of repertorising the whole case, when there is peculiar and striking symptoms or if their is any etiology which can be referred in the appropriate repertory immediately. So repertory simplifies and strengthens our selection for particular medicine.

4. Helps to find out complete symptom.
In repertory, a complete symptom with all its components can be referred to one place, especially concomitance which are scattered in several places in materia medica.

5. It promote discovery of medicine.
which one had not thought of. such symptom would remain in his mind forever.

6. Second prescription.
It suggest related remedies which could be helpful for selecting a drug for follow up or second prescription.

7. Its constant use makes the physician efficient.
By constant handling one refreshes the knowledge of M.M, difficult symptoms and medicines with different grades.

8.. It help the physician to ask intelligent questions.
Some times the patients are not able to tell their symptoms correctly or the physician not able to get the symptoms from them for the choice of medicine ,under such circumstances one can use repertory to his advantage and finally chose the medicine by asking questions guided by repertory.

9. Repertory teaches us to be careful about those symptoms       belonging to the disease [common symptoms] and to consider only those symptoms which lie outside the disease [uncommon symptoms of the disease]  

Allopaths diagnose the disease for the purpose of commencing the treatment where as homoeopaths diagnose the disease for the purpose of removing the common symptom of the disease and to consider the rest of the symptom that lie outside the path gnomonic symptom.
Repertory makes the study of M.M extremely interesting and revels more and more about uncommon symptoms. When we are in doubt a rubric, the meaning of which is not clear, we are compelled to read the materia medica for the exact meaning.

10. It help the study of Materia medica.
Take any drug and refer any section in the repertory which one wants to study and not down the presence of remedy against the given rubric.
Help in comparative study of drugs.
The intensity of symptom in a drug can be studied which help in the final selection of medicine.
Repertory expand our knowledge by giving more remedies for  keynotes.

11.To recall.
Sometimes a symptom strikes a physican, but unable to recall the medicine. In such cases he can seek the help of repertory.

12.To find out the similimum.
Symptoms are converted in to rubrics, these rubrics are located in the repertory. The repertory gives an idea about co running medicines, so the indicted medicine can be found out.

LIMITATIONS OF REPERTORY
Repertoryis basically an index, never suggest a final choice.
Different repertories have different philosophy and construction.
New additions to materiamedica cannot be accommodated in repertory.
Many rubrics are not represented well.
No guidance about potency, dose and repetition.
Nosodes and sarcodes not represented well.
If the physician makes mistakes in interpretation, and just counting the symptoms and markes, he will fail.
Use of repertory cannot be independent of knowledge of materia medica,organon or clinical subjects.

REFERENCES 
Tiwari : Essentials of repertorisation
Dhawle : Principles & practice of Homoeopathy
Dockx : Comparative repertory
Ritu : Study of repertory