As far as I can see, there are three main reasons why a boy would be circumcised:
1. Religious reasons
2. Medical reasons
3. Cultural reasons
In terms of reason 1, the mainstream religions that require their males to be circumcised are Judaism and Islam. Therefore, all Jewish and Muslim men are 'cut'. Furthermore, this means that the majority of men in the Middle East are circumcised.
Sometimes, a boy may have a medical condition where a circumcision is required. The most common such condition appears to be phimosis, where the foreskin cannot be pulled back.
Although these problems are rare, they are usually the reason behind most circumcisions in first-world countries other than the U.S.
Ask me to define 'culture' and I'd struggle to be very precise at all.
But let me give you an example: Nelson Mandela.
It was a custom for him to undergo a circumcision at the age of 16, as a sign of transition from boy to man. Mandela even had to bury his own severed foreskin in the ground as part of the ritual.
In the Philippines, boys undergo a similar cultural tradition when they hit puberty, and have their foreskins cut off too.
Now I want to make something clear:
The USA is the only developed country that circumcises most of its boys for non-religious and non-medical reasons. Our European, Asian, Australian and South-American friends do not routinely circumcise.
However, the U.S. rate of circumcision has dwindled to just 57% with the lowest rates in the south and west and higher rates in the midwest.
Later, I will make very important comparisons between the USA, a country where 80% of males are 'cut', and countries where 80% of males are 'intact'.
To conclude this section, I can summarise by telling you that on the global scale, approximately 80% of males are 'intact'.
At the time of going to print, the world's population was 6¾ billion.
So assuming there are around 3.4 billion men on the planet, that means about 2.7 billion of them have foreskins!