I am an inactive Freemason; Master Mason, Scottish Rite 32nd degree, (although I know the 33rd degree doctrine) York Rite Knight Templar, and Shriner, and my wife was an Eastern Star officer.
The men that I met involved with masonry were usually not unusual. They were everything from construction workers, police, to wealthy business owners and Judges.
I suspect most of them got involved with the idea it would lead to social and business advancement. They are not privy to any real occult secrets, as the ceremonial degrees are very heavily veiled.
I became involved with more esoteric orders that where not under the direct auspices of international freemasonry, but had very shadowy connections to whatever is behind them all. (check out lvx.org ).
I learned from higher grade members in this Order that freemasonry originally was sort of a seeding ground/sieve for more secret organizations. It may claim ancient roots, but it's current form actually was founded in the early 17th century shortly after the appearance in Europe of the "Confessio Fraternitatis R.C. ad Eruditos Europae" (Confession of the Rosicrucian Fraternity) and the "Fama Fraternitatis" (Fame of the Fraternity) of the same.
These can be read in Paul Cases "The True and Invisible Rosicrucian Order" Apparently they began appearing in intellectual circles, first in Germany around 1610, with printed versions showing up a few years later.
I was told that because of the invention of the printing press, it was decided that certain secret orders that had been around for quite some time and maintained at the same level of membership should be expanded. Freemasonry is sort of introductory secret society, and men who showed aptitude for occult ceremonial and doctrine were singled out and confidentially invited to higher, more secret orders.
Over time, it became lost to even the highest level leadership in Masonry that this was it's original purpose, but still sometimes functions in this capacity, although rarely.
Today, Masonry has been decreasing in membership for decades, and is sort of a social club for mostly elderly members who do laudable charitable works, and almost universally have no idea what the rituals, dogmas, symbols, regalia, degrees, etc mean, except for vague references in said degrees. They do understand the simple moral and ethical lessons imparted at times in the degrees, and benefit to the degree that they follow these.
At the top of this, at the 33rd degree, you use to be introduced to the "Luciferian doctrine". I've heard that the modern degree is less forthcoming, but I'm not sure how. The original, that you can read about in Albert Pikes "Morals and Dogma" basically says that Lucifer is the real God, and Jehovah is the real baddie. This cat is out of the bag, and is the main cause for criticism in modern times for obvious reasons.
Only a small percent of Scottish Rite Masons become 33 degree "Grand Inspector Generals". This seems to be only awarded after considerable time and money is given to the organization and it's charitable works. The rest of the lower grades are pretty much done by any member if he stays in a while, at least in American Scottish Rite.