Why are Secret Societies Formed? 

Secret Societies probably started shortly after man invented organized Religion.  Organized Religions are not very fond of those who do not totally agree with the so-called party line.  If a person was somewhat a Freethinker, whose beliefs were not totally in agreement with the established Religion in his or her country, he or she needed to keep these beliefs secret from the authorities of the local Religion.  Not keeping your ideas secret often leads to that person being imprisoned, tortured or killed.  Many people with beliefs different from those of the established Religion have been arrested, imprisoned, tortured until they confessed their heathen ideas and then killed in some unpleasant manner, as an example to others so others would not be so foolish as to hold an idea that did not agree with the Religion holding power at that time. Some Christians liked to tie people to a wooden post, pile combustible materials around them then set fire to the material and roast the person to death in a slow torturous process called Burning at the Stake.  Secret societies will form when the people are oppressed either by a Religion, or by a government, which at various times have been the same.

Many secret societies have existed for the sole purpose of allowing those with unacceptable ideas to express their ideas to other Freethinkers who did not always agree with what their Religion was telling them.

There has also been some secret societies that were formed as  criminal enterprises.

Freemasonry is considered by some to be a Secret Society.  This is a ridiculous idea since most Masonic Lodges are listed in the phone book and most Masons were an identifying pin, watch or ring.  If Freemasonry is attempting to be a Secret Society, it has failed in that regard.