There are a lot of demands for cram schools because, in short, entrance examinations, the dissatisfaction with public schools,and the trend of cram schools. I will explain these factors.
Entrance examination
When you want to enter a high school or university, you have to pass the entrance examination. You have to get high points in the test to enter famous schools. You have to study hard and efficiently to get high points. You can make use of cram schools to study hard and efficiently.These are one reason why there are a lot of demands for cram schools in Japan.
Educational backgrounds are taken very seriously in Japanese society, and other developed countries'. So in many countries, probably it occur that many students go to cram schools to enter famous schools.
Dissatisfaction with public schools
In public schools, teachers cannot teach only subjects but also human relationships, the respect to others, and so on. What to teach is determinated by Ministry of education. Moreover, after "Yutori kyoiku", which Ministry of education carried out, there is the critical opinion that scholastic ability decllined.
However in cram schools, teachers nearly teach only subjects because Cram schools' purpose is to make students pass entrance examinations.
So if parents want teachers to teach students subjects more thoroughly for entrance examination, cram schools may be more suitable for them.
The trend of cram schools
Many parents make their children go to cram shools now. Possibly they feel anxiety like "Is education in public schools really sufficient? ", "Only my son do not go to cram school ?". I think it is one reason why cram schools are popular that there are Japanese custom,"Harmony with others".
I think it is not important whether you go to cram schools or not, and cram schools are only one way to study more efficiently.