The Real Reason Muzan Has A Human Family In Demon Slayer
Muzan Kibutsuji is the ultimate antagonist of the "Demon Slayer" franchise. The original demon, he stands as one of the most merciless and unabashedly evil characters in the entire show. Eating people means little more than sustenance to him, and he'll do anything it takes in order to cure his weakness to the sun and become truly immortal. This is why he travels the land transforming people into demons. He believes that one day he might be able to create a demon with no weakness to sunlight.
However, there are a few times where we see the demon lord Muzan display a bit of humanity. In Episode 8, we actually see him being kind to a certain group of people: his human wife and daughter. Don't let that fool you into thinking Muzan has a softer side. Anyone who has seen a good deal of "Demon Slayer" can tell you that the kindness he shows his family is a facade. He cares for them only as tools to further his agenda and continue his quest for immortality.
Even so, it's a wonder why anyone like Muzan would bother keeping a family in the first place. Well, there actually is a very logical reason why Muzan has a human family in "Demon Slayer." In many ways, he actually needs them to survive.
Muzan keeps a family so he can stay incognito
While it is true that Muzan loves being an immortal demon, he does recognize that it has some major drawbacks. First and foremost, of course, is his weakness to sunlight. However, there's a second, more subtle weakness that comes with demonhood. It is much harder for Muzan to blend into society if he is always alone. If people found out he was a demon, no amount of shapeshifting or immortality could protect him from being hunted by a society as large as Japan. Having a wife and kid not only increases his social standing, making it easier for him to retain his cover as a human, but also allows him to draw on the resources of others.
However, this explanation is mostly speculation. In truth, Muzan's family does not appear very often in "Demon Slayer" at all. Like many of the humans Muzan has used, abused, and killed over his 1,000 years, his family is swept away as easily as they are introduced. Using his family as a cover is simply the best estimation fans could come up with based on Muzan's personality and some extraneous material (via Reddit).
The main justification for this explanation, however, is that there's really not a better one. What else would Muzan gain from a human family? He certainly doesn't need their love and affection. After all, the family we see him act so kindly toward in Episode 8 isn't even his first family.
Muzan has had many families over the years
It's true. Muzan has had more than one family over the course of his millenia-long life. This includes the birth family he had before he became a demon. In addition to them, Muzan took five wives. Almost none of these family members met very pleasant ends.
In the case of Muzan's biological family, it is implied that they survived the ages as the Ubuyashiki clan. Not only do certain Ubuyashikis resemble Muzan to the point that they could be siblings, but the entire Ubuyashiki bloodline had been cursed with short lifespans until the Demon Slayer Corps finally defeated Muzan at the end of the series (via Kimetsu no Yaiba wiki). Ironically, the Ubuyashikis would come to be some of Muzan's greatest enemies, as Kagaya Ubuyashiki became the 97th head of the Demon Slayer Corps.
Toward his many wives, Muzan is abusive to say the least. He mistreated each of his wives so horrendously that he drove each one to suicide in turn. Obviously, that did not dissuade him from continuing to attract innocent women. "Demon Slayer" never covers the eventual fate of that wife and daughter seen in Episode 8, but we can only hope they managed to survive beyond Muzan's eventual death.