History of Taqleed Mazhab Imam necessity Tareekh Evolution developement summary

The great scholar of Islamic history, reformist and king Faisal Award winner Maulana Abul Hasan Ali Nadvi ( Ali Miyan) R.A. describe this situation of second hundred hijri in these words in his book Tareekh e dawat o Azeemat vol 1(page 80-81) {its English translation The saviors of Islamic sprit Vol1 .”}

 “Islam has expanded  to a vast and fertile areas of Syria, IRAQ, Egypt and other African countries,Iran and central Asia. Administration of country, Maashrat,(Society) business, has reached to a complex situation. And it was urgent need to extraction and ISTANBAT and interpretation of the fine details of these principles. For this important job a very deep knowledge and understanding of Quran and sunnat and its practical application to the current situation were required.”

Tareekh e dawat o Azeemat vol 1(page 80-81) {its English translation The saviors of Islamic sprit Vol1 .}

Need for permanent mechanism and its realization by Salfus Salihin
The Ulema and Mashaikh realized the situation in time and hundreds of scholar started devouting themselves for this noble cause. They took it as a permanent job. They were not only the famous four Imam but so many others either working at different Islamic centers.May Allah give them best reward.
Ali Miyan  further writes in his book
“It was bounty from Allah and pinnacle of destiny (IQBAL MANDI) of ummate muslima that muslims got people of special qualities for this important job.
 Tareekh e dawat o Azeemat vol 1(page 80-81) {its English translation The saviors of Islamic sprit Vol1 .}
.....................They were outstanding in knowledge of quran and sunnat, intellectuality, piousness, piety,and Ikhlas. Thousands and thousand of such person devouted their life for this noble and essenitial cause. There were more scholar group but out of these four distinguished Imam emerged whose Jurisprudence ruling were compiled in a systematic way and their ruling is still alive and popular. These are Imam abu hanifa (Died 150 hijri,followed by nearly 47 % of muslims). Imam shafai (Died 204 H followed by 29 %) Imam malik (Died 179 H followed by 16 %),Imam Hanbal (Died 241H,followed by 3 %) . They comprise nearly 96 % of the total Sunni muslim population of the world.
            These Imams got very competent and  uncomparable quality disciple.Imam Abu Haneefa got Imam Abu yusuf,and Imam Muhammad,Imam Malik got Abdullah ibne wahab,ibnil qasim,Imam shafai got Muzni and Rabi, and Imam Ahmad got Person like Ibne Qadama.
Benefit of this system 
They not only derived ruling but also compiled the basis of ruling from quran and sunnat, Deviced principles that helped further generation of ulema and scholar to derive rules from Quran and sunnat and to guide muslims on all aspect of life, including finance and administration etc.

1This estimation of Hanafi shafaee Maliki Hanbali is refrenced to the document THE 500 MOST INFLUENTIAL MUSLIMS 2010 edition by The Royal Islamic Strategic Studies Centre Page no.16&18 and of Wikipedia and merely estimation. Some of the other sources have estimated Hanafis more than 50% of the sunni muslims.)