it's not just a manga thing, it happens in real life a lot. since same sex marriage isn't allowed in japan, the older one will adopt the younger one so that they're officially family and whatnot
Aside fr "making it official", being a household has legal and social benefits such as taxes, insurance, etc.
But if one of them dies or gets really sick their "roommate" would have no legal rigHts to help them or make decisions for them, and would lose everything to the "relatives" if one of them passed away. With the adoption they are legally family and the lover is the next of kin just like they would be if they'd been able to get married.
I hope you know Japan doesn't allow same-sex marriages. Thus, the only form of actually "getting married" for homosexuals would be adoption. The partner would be able to handle all the legal matters, including inheritance, just in case the other partner (the adopter) dies. It's basically the same as same-sex marriage, except legal and with the term "adoption." That's why many long-term homosexual couples do this.
Second, being roommates does not have the same legal benefits that adoption does.
I always think it weird when manga artist think the only way to become a family is by adopting their lover. It makes it even more weird! Just live together and say you two are roommates. Sheesh.
I liked these stories. But I wish they weren't so seemingly related to each other.