For BTS : (I copy paste one of my old comments honestly because this question has been asked seeral times).
BTS members are really relatable and are all boys next door. ARMY love their chemistry, their humour and their accessibility. They spend a lot of time interacting with theirs fans.
There are other things that set BTS apart. The biggest of these is the artistic control they have over their music. Many of the songs are written in part or whole and produced in part or whole by them. Much of their music is unique, is not your tradtional love song and holds a lot of meaning. Many ARMY talk about how BTS' message caught their eye and brought them into the fandom.
They deal with social/political issues in theirs songs and they are mainly talking about theirs personal struggles. That’s why their music holds a strong emotional connection with the fans. Their discography conveys a tragic yet comforting view of human existence : everything is ephemeral and never last. Happiness don’t exist only moments of joy. Yet, BTS never fail to comfort their audience and give them hope. For example in the same album, Butterfly is about the fear of loosing a loved one and Run is about the acceptance of the end of a relationship.
BTS depict a lot of relatable struggles and share a cure at the same time. In a way, they are the cure.
So their message is universal and for a lot of ARMY, theirs songs are their home.
Their biggest strength is theirs performances. Even people who aren’t ARMYs say it. They never perform twice the same way. They are versatile performers, charismastic with a different charm for a lot of them and they are very energetic sometimes wild.