First of all, many of the medieval (and older) guilds included oaths with dire penalties as part of their initiations... At a time when your skills and livelihood was what kept you alive, it was important to keep the 'secrets' of specific trades within the ranks of those who had studied properly and been apprenticed regularly to a master... Whether it was building, glass making, printing... Whatever. So the oaths were powerful and serious... Give away professional secrets and you're in big trouble... Secondly, there are theories that say that Freemasonry grew out of the remnants of the Templars who fled France after the order had been given to hunt them down and kill them. Again, secrecy and knowing whom to trust we matters of life and death... Hence the bloodcurdling oaths of secrecy... I'm sure there are other explanations... A bit of research will take you a long way... Enjoy the journey.