This sort of adoption was quite common in Japan all the way through the mid-20th century, and was not restricted to samurai--it was practiced by all classes/castes. It was a way for a family that did not have a male heir, to obtain a male heir. The practice, and the adopted son, is called "mukoyoshi". Basically, inheritance in Japan at that time typically took the form of primogeniture--the eldest son inherited the bulk of the family property. if a family had only daughters, then the family would adopt a man to marry one of their daughters. He would take her last name, and he would inherit the family property in due time. This was an appealing situation for a man who was not his own family's eldest son, because he would inherit more property than otherwise. So typically someone who would accept becoming a mukoyoshi would be a younger son--an elder son, who already had property to inherit, wouldn't want that situation. It was also very appealing to the girl involved, because she could then stay in her family home, with her own parents. The most common marital situation during that era in Japan was that a girl would marry and move to her husband's home (patrilocality), where she would be under the authority of her mother-in-law within the home, and in any case, would have less of a support network because she wouldn't be in the environment in which she grew up. Depending on the personality of the mother-in-law, the girl could be very abused or mistreated. She could typically expect better treatment from her own parents than from her in-laws. In cases where a family had neither son nor daughter, it was a common practice to adopt either a son or daughter from a relative, so that the family would have an heir, so that, too, was an option. Because such practices were common and accepted, it meant that there was much less stigma or fear of not having a son. In a society like, say, China, it was enormously important to have a son, and there was no substitute. But in Japan, this was an accepted substitute and so while a family might prefer to have a son, it wasn't a tragedy or enormous hardship to not have a son. They could still keep their property in the family. People would regard a woman who married a mukoyoshi as being quite fortunate, but also perhaps sometimes a bit spoiled, because she hadn't gone through the really grueling circumstances that most women went through, of having to live under the thumb of their husband's parents. And a man who became a mukoyoshi might be seen as a bit weaker in position. But that all depended upon the particular circumstances. One way this was also convenient was that if, say, a man was a skilled artisan, he could adopt a favorite student and have the student marry his daughter and inherit his property. The family adopting the man might actually undertake to educate him from a young age, depending on the circumstances.