In Albania it establishes the responsibility of one party to avenge the death of the other. The idea is that an enemy would be less likely to attack you if he knew that another man is going to kill him and his whole family for it. Do the Scots-Irish of Appalachia have a similar ceremony?
As a Scots-Irishman of Appalachia, I have to say...kinda. It's usually an informal thing, there's no ceremony. But certainly two men can come to care for each other in such a way they refer to each other as mutual brothers. Often, this would include common attendance at family events and other things. We like adopting people into our families!
As far as the rule of blood, it was an ancient Cherokee practice. If one person murders another, the offended clan is awarded the duty to kill the offender...or the offender's next-of-kin.